Introduce build minutes (again)

This brings back a change introduced by: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/965, but then reverted.

Right now:

1. limits are stored as minutes,
2. accounting is done in seconds.

@ayufan ping me once merged to write the docs.

added 189 commits

• 2b691f6d...85ef6a6f - 186 commits from branch master
• 71165067 - Merge branch 'introduce-build-minutes' into 'master'
• 08dc2c7c - Rename shared_runners_minutes to be shared_runners_seconds when calculating data
• 39ae232a - Return int for minutes when calculated from seconds

Compare with previous version

enabled an automatic merge when the pipeline for 39ae232a succeeds

merged

mentioned in commit 57293569

mentioned in merge request !965 (merged)

Picked into 8-16-stable-ee, will go into 8.16.0-rc5

removed ~149424 label

mentioned in merge request !1081 (merged)

mentioned in commit 86bc9cd9

mentioned in merge request !6872 (merged)

mentioned in issue #7246

added Enterprise Edition label

mentioned in merge request !29950 (merged)