Updating the docs to discourage HTTP usage when there's no downside to HTTPS usage.

Now that the k8s service is being deprecated in favor of the
Clusters integration, copy all info from the old to the new page.

Closes gitlab-org/gitlab-ee#4419

[ci skip]

Remove duplicate list item in "General Guidelines" Section.

Should fix https://gitlab.com/gitlab-org/gitlab-runner/issues/2652

Adds sorting to deployments API through the `order_by` and sort
`fields`.

Adds `#build_notification_recipients` to `NotificationRecipientService`
that returns the `NotificationRecipient` objects in order to be able to
access the new attribute `reason`.

This new attribute is used in the different notifier methods in order to
add the reason as a header: `X-GitLab-NotificationReason`.

Only the reason with the most priority gets sent.

[10.3] Migrate `can_push` column from `keys` to `deploy_keys_project`

See merge request gitlab/gitlabhq!2276

(cherry picked from commit f6ca52d31bac350a23938e0aebf717c767b4710c)

1f2bd3c0 Backport to 10.3

[10.3] Fix path traversal in gitlab-ci.yml cache:key

See merge request gitlab/gitlabhq!2270

(cherry picked from commit c32d0c6807dfd41d7838a35742e6d0986871b389)

df29094a Fix path traversal in gitlab-ci.yml cache:key

Converted from type to "stage" due to depreciation.

Signed-off-by: JJ Asghar <jjasghar@gmail.com>

Enables project milestone deletion via DELETE /projects/:id/milestones/:milestone_id