Update CHANGELOG.md for 10.3.0

CHANGELOG.md

changelogs/unreleased/13634-broadcast-message.yml

...n-on-create-new-pipeline-in-case-of-validation-errors.yml

...elogs/unreleased/15774-fix-39233-500-in-merge-request.yml

...nreleased/18040-rubocop-line-break-after-guard-clause.yml

...mize-branch-name-when-using-create-branch-in-an-issue.yml

...el-slash-command-limit-autocomplete-to-applied-labels.yml

.../unreleased/28377-add-edit-button-to-mobile-file-view.yml

...-checking-on-checklist-if-potential-spam-was-detected.yml

...sed/32057_support_ordering_project_notes_in_notes_api.yml

changelogs/unreleased/32098-pipelines-navigation.yml

changelogs/unreleased/32878-merge-request-from-email.yml

...lization-support-for-prometheus-service-configuration.yml

changelogs/unreleased/34600-performance-wiki-pages.yml

...leased/35385-allow-git-pull-push-on-project-redirects.yml

changelogs/unreleased/35616-move-k8-to-cluster-page.yml

changelogs/unreleased/35724-animate-sidebar.yml

changelogs/unreleased/36400-trigger-job.yml

changelogs/unreleased/37354-pipelines-update.yml

...nreleased/38032-deploy-markers-should-be-more-verbose.yml