release-23.2: sql: mark implicit transactions with AOST as read-only

[michae2]

Backport 2/2 commits from #120097.

/cc @cockroachdb/release

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sql: mark implicit transactions with AOST as read-only

In handleAOST we were setting the transaction AOST but not marking it as read-only. It needs to also be marked as read-only to disallow mutation statements and locking statements.

Fixes: #120081

Release note (sql change): Mutation statements such as UPDATE and DELETE as well as locking statements such as SELECT FOR UPDATE are not allowed in read-only transactions or AS OF SYSTEM TIME transactions. Fix an oversight where we were allowing mutation statements and locking statements in implicit single-statement transactions using AS OF SYSTEM TIME.

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sql: disallow SET transaction_read_only = false during AOST txn

Fixes: #44200

Release note (bug fix): Fix a bug in which it was possible to SET transaction_read_only = false during an AS OF SYSTEM TIME transaction.

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Release justification: add a missing check preventing undefined combination of AOST and mutation statements.

[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).
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• 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).
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• 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 

[michae2]

Hold off for a sec, I need to fix it.