Add uniqueness constraint on workspaces name for the pg backend

[remilapeyre]

While it should not be possible to create two workspaces with the same name as the index should be created by the database administrator when skip_index_creation is used, this additional constraint will make it impossible to do so even if the index is missing.

I also reworded the documentation for the skip_... options which was inaccurate.

Discussion at #29034

[jbardin]

Thanks @remilapeyre!

[github-actions]

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