Merge branch '38464-k8s-apps' of https://gitlab.com/gitlab-org/gitlab-ce into add-ingress-to-cluster-applications

Fix spec. spec/serializers/cluster_application_entity_spec.rb and spec/serializers/cluster_entity_spec.rb

Resolve "Fix GKE wording"

Closes #39648

See merge request gitlab-org/gitlab-ce!15204

Merge branch '39791-when-reopening-an-issue-the-mattermost-notification-has-no-context-to-the-issue' into 'master'

Resolve "When reopening an issue the Mattermost Notification has no context to the issue"

Closes #39791

See merge request gitlab-org/gitlab-ce!15218

Merge branch '37442-api-branches-id-repository-branches-is-calling-gitaly-n-1-times-per-request' into 'master'

Fetch the merged branches at once in the `/projects/:id/repository/branches` endpoint

See merge request gitlab-org/gitlab-ce!15215

Support custom attributes on groups and projects

See merge request gitlab-org/gitlab-ce!14593

Add changes_count to the merge requests API

See merge request gitlab-org/gitlab-ce!15178

This reverts commit 36d69130.

First iteration of Move Kubernetes from service to Cluster page

See merge request gitlab-org/gitlab-ce!15061

[CE backport] Saved configuration for issue board

See merge request gitlab-org/gitlab-ce!15108