Also crosslink from issue template and expand deprecation
template information. Removes end of support milestone
from removal template, as it is not needed.

Changelog: added
EE: true

This reverts commit 32c2a606b6d92d7ac601ddf7fe33fb0c9302da95.

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

Changelog: deprecated

Also refine deprecation items for each

- Update templates
- Update occurences of contributing/#breaking-changes

The change is a breaking change, but the deprecation
and removal show breaking_change: false

This follows up on a number of issues we found when direct uploads
became the default:

- Buckets may not have delete permissions
- Using a bucket prefix no longer works

We include a workaround to make background uploads work again.

Relates to items discussed in:
- https://gitlab.com/gitlab-org/gitlab/-/issues/365810
- https://gitlab.com/gitlab-org/gitlab-runner/-/issues/26869

This reverts commit b46612e2.