Update CHANGELOG.md for 12.8.0

[ci skip]

changelogs/unreleased/refactoring-entities-file-8.yml

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-title: Separate snippet entities into own class files
-merge_request: 24183
-author: Rajendra Kadam
-type: added

changelogs/unreleased/refactoring-entities-file-9.yml

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-title: Separate protected and issuable entities into own class files
-merge_request: 24221
-author: Rajendra Kadam
-type: added

changelogs/unreleased/refactoring-entities-file-final.yml

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-title: Separate project entity into own class file
-merge_request: 25297
-author: Rajendra Kadam
-type: added

changelogs/unreleased/refactoring-entities-file.yml

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-title: Seprate 5 classes in separate files from entities
-merge_request: 23299
-author: Rajendra Kadam
-type: added

changelogs/unreleased/remove-issue_link_types.yml

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-title: Expose issue link type in REST API
-merge_request: 24175
-author:
-type: added

changelogs/unreleased/remove-multiple-milestone-mess.yml

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-title: Remove unnecessary milestone join tables
-merge_request: 25198
-author:
-type: changed

changelogs/unreleased/remove-self-monitoring-feature-flag.yml

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-title: Remove self monitoring feature flag
-merge_request: 23631
-author:
-type: other

changelogs/unreleased/remove_milestone_id_from_epics3.yml

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-title: Remove milestone_id from epics
-merge_request: 23282
-author: Lee Tickett
-type: other

changelogs/unreleased/rename-self-monitoring-project.yml

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-title: Rename 'GitLab Instance Administration' project to 'GitLab self monitoring' project
-merge_request: 23182
-author:
-type: changed

changelogs/unreleased/reorder-signup-omniauth-options.yml

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-title: Reorder signup omniauth options
-merge_request: 23082
-author:
-type: changed

changelogs/unreleased/reschedule-issue-tracker-migration.yml

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-title: Migrate issue tracker data to data field tables
-merge_request: 24076
-author:
-type: other

changelogs/unreleased/resolve_gitlab_issue_196651.yml

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-title: Replace underscore with lodash for ./app/assets/javascripts/serverless
-merge_request: 25011
-author: Tobias Spagert
-type: other

changelogs/unreleased/resolve_gitlab_issue_196658.yml

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-title: Replace underscore with lodash for ./app/assets/javascripts/error_tracking
-merge_request: 25143
-author: Tobias Spagert
-type: other

changelogs/unreleased/rest_api_log_last_activity_on.yml

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-title: Log user last activity on REST API
-merge_request: 21725
-author:
-type: fixed

changelogs/unreleased/revert_rename_services_template_to_instance_migration.yml

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-title: Revert rename services template to instance migration
-merge_request: 24885
-author:
-type: fixed

changelogs/unreleased/rk4bir-master-patch-88996.yml

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-title: Migrate from class .fa-spinner to .spinner in app/assets/javascripts/gfm_auto_complete.js
-merge_request: 25039
-author: rk4bir
-type: changed

changelogs/unreleased/rs-scheduled-pipeline-play-api.yml

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-title: Add API to "Play" a scheduled pipeline immediately
-merge_request: 23718
-author:
-type: added

changelogs/unreleased/rubyzip-qa-update.yml

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-title: Update rubyzip gem in qa tests to 1.3.0 to fix CVE-2019-16892
-merge_request: 24119
-author:
-type: security

changelogs/unreleased/sarnold-197129-graphql-feature-flag.yml

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-title: Add ability to hide GraphQL fields using GitLab Feature flags
-merge_request: 23563
-author:
-type: added

changelogs/unreleased/sdesk-name-handler.yml

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-title: Added migration which adds project_key column to service_desk_settings.
-merge_request: 24063
-author:
-type: added