Bump buildbot to v3.11.8 release

Bump buildbot to v3.11.8 release

Bug fixes
---------

- Made Tags column in Builders page take less space when there are no tags.
- Fixed cropped change author avatar image in web UI.
- Correctly pluralize build count in build summaries in the web UI.
- The change details page no longer requires an additional mouse click to show the change details.
- Fixed showing misleading "Loading" spinner when a change has no builds.
- Fixed too small spacing in change header text in web UI.
- Fixed showing erroneous zero changes count in the web UI when loading changes.
- Cleaned up build and worker tabs in builders view in web UI.
- Improved visual separation between pending build count and build status in trigger build steps in web UI.

Changes
-------

- Buildbot has migrated to ``quay.io`` container registry for storing released container images.
  In order to migrate to the new registry container image name in ``FROM`` instruction in Dockerfiles
  needs to be adjusted to ``quay.io/buildbot/buildbot-master`` or ``quay.io/buildbot/buildbot-worker``.
- The list of supported browsers has been updated to Chrome >=80, Firefox >= 80, Edge >=80,
  Safari >= 14, Opera >=67.

Features
--------

- Add a "Workers" tab to the Builder view listing workers that are assigned to this builder (:issue:`7162`)
- The text displayed in build links is now configurable and can use any build property.
  It was showing build number or branch and build number before.
- Changes and builds tables in various places in the web UI now have a button to load more items.

Signed-off-by: Petr Štetiar <ynezz@true.cz>