[Merged by Bors] - Congruence subgroup namespace

[CBirkbeck]

Put congruence subgroups into their own namespace, so that things like Gamma don't live in the root namespace.

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• depends on : [Merged by Bors] - Chore: Rename files with Eisenstein series stuff. #14572

[github-actions]

PR summary 6b053dd002

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

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Declarations diff

No declarations were harmed in the making of this PR! 🐙

You can run this locally as follows

## summary with just the declaration names:
./scripts/no_lost_declarations.sh short <optional_commit>

## more verbose report:
./scripts/no_lost_declarations.sh <optional_commit>

[loefflerd]

LGTM. I don't know why the bot removed the "blocked by other PR" label, though.

Maintainers: please merge this once #14572 is in.

maintainer merge

[github-actions]

🚀 Pull request has been placed on the maintainer queue by loefflerd.

[riccardobrasca]

Thanks!

bors d+

[mathlib-bors]

✌️ CBirkbeck can now approve this pull request. To approve and merge a pull request, simply reply with bors r+. More detailed instructions are available here.

[mathlib-bors]

Canceled.

[CBirkbeck]

bors r+

[mathlib-bors]

Pull request successfully merged into master.

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