chore(WhatsNew): rename whatsnew to #whats_new

[JovanGerb]

This PR renames whatsnew to #whats_new, following the naming convention.

See https://leanprover.zulipchat.com/#narrow/channel/287929-mathlib4/topic/.60whatsnew.20in.60.20rename/with/615003669

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[github-actions]

PR summary abe92ef615

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 (regex)

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

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.

Declarations diff (Lean)

✅ Lean-aware diff — post-build, computed from the Lean environment (commit abe92ef).

• +2 new declarations
• −1 removed declarations

-Mathlib.WhatsNew.commandWhatsnewIn__
+Mathlib.WhatsNew.oldStx
+Mathlib.WhatsNew.«command#whats_newIn__»

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No changes to strong technical debt.

No changes to weak technical debt.

Current commit abe92ef615
Reference commit ac10dc7e9a

This script lives in the mathlib-ci repository. To run it locally, from your mathlib4 directory:

git clone https://github.com/leanprover-community/mathlib-ci.git ../mathlib-ci
../mathlib-ci/scripts/reporting/technical-debt-metrics.sh pr_summary

• The relative value is the weighted sum of the differences with weight given by the inverse of the current value of the statistic.
• The absolute value is the relative value divided by the total sum of the inverses of the current values (i.e. the weighted average of the differences).

[thorimur]

Once you attach the docstring to the old syntax to get CI passing, this looks good, thanks! :)

maintainer delegate

[github-actions]

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