sql: session var disable_drop_virtual_cluster

[knz]

Informs #106068.
Epic: CRDB-29380

This replaces disable_drop_tenant, together with the cluster setting sql.drop_virtual_cluster.enabled (replaces
sql.drop_tenant.enabled).

We are OK with this breaking change since the feature was not yet exposed to end-users.

Release note: None

[cockroach-teamcity]

This change is 

[yuzefovich]

Reviewed 12 of 12 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @stevendanna)

[knz]

TFYR!

bors r=yuzefovich

[craig]

Merge conflict.

[knz]

bors r=yuzefovich

[craig]

Build failed (retrying...):

• Bazel Essential CI (Cockroach)

[craig]

Build succeeded:

• Bazel Essential CI (Cockroach)

[blathers-crl]

Encountered an error creating backports. Some common things that can go wrong:

1. The backport branch might have already existed.
2. There was a merge conflict.
3. The backport branch contained merge commits.

You might need to create your backport manually using the backport tool.

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error creating merge commit from ab28123 to blathers/backport-release-23.1-106121: POST https://api.github.com/repos/cockroachdb/cockroach/merges: 409 Merge conflict []

you may need to manually resolve merge conflicts with the backport tool.

Backport to branch 23.1.x failed. See errors above.

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