Remove reference to Starter tier from Admin Area page.

Add and remove needed tier badges for documentation page.

This updates the changelog documentation to cover the new changelog
workflow, and Danger to check for this new workflow correctly.

Related to: https://gitlab.com/gitlab-org/gitlab/-/issues/325900
Changelog: added
MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/61775

Co-authored-by: Achilleas Pipinellis <axil@gitlab.com>
Signed-off-by: Takuya Noguchi <takninnovationresearch@gmail.com>

This reverts merge request !61758

Remove outdated GitLab version references from the doc.

Remove outdated version reference from doc page.

Prior to this, the external pipeline validator would reject a pipeline
if the service returned any 4xx error code, but the behavior was
different for GitLab.com, where only a 406 error code would be used to
reject.

We simplify the handling of this error code by making 406 the only error
code to signify a rejection. This not only makes it easier to
understand, but it also makes the behavior consistent from one GitLab
instance to another.

Changelog: changed

These pages all mentioned 'master' and needed to be updated to point
to either the documentation for default branches, or to use 'main'
instead.

With removing Unicorn support in 14.0, UnicornSampler is no longer
needed.
Raindrops was only used with Unicorn, so it could be removed too.
Update the docs related to the change.

Changelog: removed