backupccl: ignore all dropped descriptors during backup

[adityamaru]

Previously, descriptors that were resolved at EndTime
and were in a DROP state were not treated uniformly. While
we ignored table descriptors, we continued to backup database,
schema and type descrpitors.

This resulted in atleast two bugs:

1. If a database descriptor was in a dropped state, and a new
   descriptor with the same name was created then a BACKUP DATABASE
   of the new database would fail during resolution.

2. A cluster backup of the above state would succeed, but since it
   would include duplicate entries for the same name database, the restore
   of such a backup would fail.

This change unifies the behaviour by ignoring all DROP descriptors seen
by the backup at EndTime. A follow up PR will teach restore to ignore
all dropped descriptors so as to allow users with "corrupt" backups as
explained in 2) to be able to restore.

Informs: #76517

Release note (bug fix): Backup incorrectly backed up database, schema,
and type descriptors that were in a DROP state at the time the backup was
run. This bug resulted in the user being unable to backup and restore if
their cluster had dropped and public descriptors with colliding names.

[cockroach-teamcity]

This change is 

[ajwerner]

Reviewed 6 of 6 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @dt and @msbutler)

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pkg/ccl/backupccl/backupresolver/targets.go, line 341 at r1 (raw file):

		if _, ok := alreadyRequestedDBs[dbID]; !ok {
			desc := r.DescByID[dbID]
			// Verify that the database is in the correct state.

Note that we do this rather than just filtering because we already resolved this database by ID above?

Code quote:

te.

[stevendanna]

LGTM

[ajwerner]

bors r=ajwerner,stevendanna

[craig]

Build failed (retrying...):

• GitHub CI (Cockroach)

[craig]

Build succeeded:

• GitHub CI (Cockroach)

[adityamaru]

blathers backport 21.2

[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 451274b to blathers/backport-release-21.2-76635: 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 21.2 failed. See errors above.

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