release-23.2: storage: expose new compaction concurrency env var

[blathers-crl]

Backport 1/1 commits from #113052 on behalf of @jbowens.

/cc @cockroachdb/release

---

Add a new COCKROACH_COMPACTION_CONCURRENCY environment variable to control the
maximum number of concurrent compactions that a single store will schedule.
This environment variable will supersede the old COCKROACH_ROCKSDB_CONCURRENCY
environment variable which was undocumented and was unfortunately named.

Epic: none
Release note (ops change): Introduced a new documented environment variable
that allows an operator to configure the compaction concurrency.

---

Release justification: Small new product addition to allow us to document the compaction concurrency environment variable in 23.2.

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Backports should only be created for serious
  issues or test-only changes.
• Backports should not break backwards-compatibility.
• Backports should change as little code as possible.
• Backports should not change on-disk formats or node communication protocols.
• Backports should not add new functionality (except as defined
  here).
• Backports must not add, edit, or otherwise modify cluster versions; or add version gates.
• All backports must be reviewed by the owning areas TL and one additional
  TL. For more information as to how that review should be conducted, please consult the backport
  policy.

If your backport adds new functionality, please ensure that the following additional criteria are satisfied:

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters. State changes must be further protected such that nodes running old binaries will not be negatively impacted by the new state (with a mixed version test added).
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.
• Your backport must be accompanied by a post to the appropriate Slack
  channel (#db-backports-point-releases or #db-backports-XX-X-release) for awareness and discussion.

Also, please add a brief release justification to the body of your PR to justify this
backport.

[cockroach-teamcity]

This change is 

[nicktrav]

LGTM for EM approval ✅.

Deferring the GitHub approval to the second TL reviewer.

[sumeerbhola]

Reviewed 5 of 5 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @itsbilal and @williamkulju)

[jbowens]

TFTR!