release-23.1: sql/license: Fix data race in license/enforcer.go

[blathers-crl]

Backport 1/1 commits from #130036 on behalf of @spilchen.

/cc @cockroachdb/release

---

There is a data race in TestQueryCache related to license/enforcer.go. This test is unique because it runs parallel tests that start a mock server, going through the SQL startup code. Since the license enforcer is a singleton, each parallel goroutine shares the same enforcer object. This commit addresses two issues:

1. The Enforcer.db variable can be set twice and is otherwise unprotected. Since the db is only needed during setup, I’ve removed the variable from the struct for now. I may need to reintroduce it later, but I’ll ensure it doesn’t break this test when I do.

2. The creation of the singleton in GetEnforcerInstance can cause a data race on the instance variable. While there is a once.Do call to generate the singleton, there was an unnecessary instance == nil check before it. I’ve removed this check to prevent the data race.

Epic: CRDB-39988
Closes: #130034
Release note: None

---

Release justification: This work is part of the CockroachDB core deprecation.

[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. 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.

[blathers-crl]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[cockroach-teamcity]

This change is