changefeedccl: db level feeds: disable db level feed creation in 26.2

[aerfrei]

We accidentally re-enabled DB-level changefeed creation in 26.2. PR #159301 re-enabled the feature before it was ready to ship and should not have been merged yet (it reverted the disable from #159000).

We need to disable it in 26.2 the same way we did for 25.4 and 26.1 (see #158386).

Epic CRDB-1421

Jira issue: CRDB-62110

[blathers-crl]

cc @cockroachdb/cdc

[blathers-crl]

Hi @aerfrei, please add a branch-* label to identify the earliest affected branch for this release-blocker

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