[Merged by Bors] - chore: protect congr_arg's and congr_fun's

[mattrobball]

We already have congr_arg and congr_fun in the root namespace.

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[grunweg]

Thanks for the clean-up! All these theorems are already namespaced, but protecting them probably doesn't hurt.
Feel free to maintainer merge on my behalf once you have fixed the merge conflicts.

[github-actions]

PR summary bec3ebb160

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/declarations_diff.sh <optional_commit>

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

[grunweg]

Given CI passes, let me
maintainer merge
myself.

[github-actions]

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

[kim-em]

bors merge

[mathlib-bors]

Pull request successfully merged into master.

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