feat(RingTheory/RamificationInertia/Ramification): ramification index is invariant under a group action

[tb65536]

This PR proves that ramification index is invariant under a group action. We already have this for the old definition ramificationIdx, but we will need this new version for ramificationIdx' for the upcoming refactor.

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[github-actions]

PR summary 956067c5e4

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

+ ramificationIdx'_smul

You can run this locally as follows

## from your `mathlib4` directory:
git clone https://github.com/leanprover-community/mathlib-ci.git ../mathlib-ci

## summary with just the declaration names:
../mathlib-ci/scripts/pr_summary/declarations_diff.sh <optional_commit>

## more verbose report:
../mathlib-ci/scripts/pr_summary/declarations_diff.sh long <optional_commit>

The doc-module for scripts/pr_summary/declarations_diff.sh in the mathlib-ci repository contains some details about this script.

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No changes to strong technical debt.

No changes to weak technical debt.