sql: don't allow schema changes in weak isolation transactions

[nvanbenschoten]

Until we can make this safe, don't allow it.

We used to have the following logic, which we can model this change on:

cockroach/pkg/sql/lease.go

Lines 320 to 325 in f0ac764

	// Use SERIALIZABLE transactions so that the ModificationTime on the
	// descriptor is the commit timestamp.
	if txn.Isolation() != enginepb.SERIALIZABLE {
	return pgerror.NewErrorf(pgerror.CodeInvalidTransactionStateError,
	"transaction involving a schemas change needs to be SERIALIZABLE")
	}

Jira issue: CRDB-26563

Epic CRDB-26546

[rafiss]

Prior to https://github.com/cockroachdb/cockroach/pull/32860/files#diff-2312a3957c7ab7f60a52d6a6f73b4f52b5265a5889b7872e49970c6e40547a8c this was done with a check in lease.go. That will return an error at the time the schema change is executed. Making a check proactively may not be easy, though we should investigate a bit.