release-26.2: backup: clear default privileges on database and schema descriptors during restore

[spilchen]

Backport 2/2 commits from #165997 on behalf of @spilchen.

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During non-cluster restore, default privileges on database descriptors were carried over verbatim from the backup, potentially referencing users that don't exist on the target cluster. Clear them alongside regular privileges, which are already reset to defaults.

The second commit extends the first fix to also clear default privileges on schema
descriptors during non-cluster restore. Schemas can also carry default
privileges referencing users that don't exist on the target cluster.

Closes #164961
Epic: none
Release note (bug fix): Fixed a bug where restoring a database backup containing default privileges referencing non-existent users would leave dangling user references in the restored database descriptor.

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Release justification: fix GA blocker

[blathers-crl]

⚠️ End-of-Life Version Warning

This backport targets 26.2, which is an End-of-Life (EOL) version.

Please verify that backporting to this EOL version is intentional and appropriate. EOL versions no longer receive maintenance updates according to our support policy.

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Thanks for opening a backport.

Before merging, please confirm that the change does not break backwards compatibility and otherwise complies with the backport policy. Include a brief release justification in the PR description explaining why the backport is appropriate. All backports must be reviewed by the TL for the owning area. While the stricter LTS policy does not yet apply, please exercise judgment and consider gating non-critical changes behind a disabled-by-default feature flag when appropriate.

[cockroach-teamcity]

This change is 

[msbutler]

hey @andrew-r-thomas , could you give this PR a close read to ensure it doesn't affect your restore with grants work? in addition, perhaps we should write a test to understand how restore with grants interacts with default privs.

[andrew-r-thomas]

@msbutler currently restore with grants leaves default privs completely untouched (and in fact my understanding assumption was that they were being cleared by the function being modified in this PR), so this shouldn't have any impact on the feature, if anything it helps, restore with grants is set up to assume that it has a nice fresh empty descriptor (wrt privs) to play with in publishDescriptors. Though potentially restoring default privs could go on our list of possible improvements to the feature if we get product/customer signal that it would be useful.

[msbutler]

@andrew-r-thomas let me be a bit more specific: suppose user max has default select privs on all tables in database foo. Suppose we backup the cluster and then run restore foo (i.e. no with grants) in a new cluster with max in it. Before this PR merged, i think max in the restored cluster would inherit the default select priv on foo. This was a bug. Now max does not inherit the select priv.

I think what you're saying is that even with restore with grants, max should not have select privs on database foo. correct?