restoreccl: insert system.namespace entry for synthetic public schemas during restore

[RichardJCai]

Release note (bug fix): Previously during restore we wouldn't insert a system.namespace
entry for synthetic public schemas.

Resolves #73535

[cockroach-teamcity]

This change is 

[RichardJCai]

TFTR!
bors r=dt

[craig]

Build succeeded:

• GitHub 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 483e4d9 to blathers/backport-release-21.1-73875: 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 21.1.x failed. See errors above.

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error creating merge commit from 483e4d9 to blathers/backport-release-21.2-73875: 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.x failed. See errors above.

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