release-20.2: kv: allow manual GC that does not advance threshold

[jseldess]

PR: cockroachdb/cockroach#60745

From release notes:

Manually enqueueing a range in the GC queue now properly respects the SkipShouldQueue option. This can be useful to force the GC of a specific range. [#60745][#60745] {% comment %}doc{% endcomment %}

[jseldess]

@taroface, not sure if this requires any docs or if it's rather related to troubleshooting guidance we could/should provide. CC @mwang1026, @nvanbenschoten, since this comes out of KV.

[nvanbenschoten]

I do not think this needs documentation. It was primarily added as a mechanism for CRDB engineers to use during support escalations.

[jseldess]

Thanks, @nvanbenschoten!