chore: deprecate Data/Tree/RBMap

[grunweg]

It is empty now.

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[github-actions]

PR summary 2b4c6152f1

Import changes exceeding 2%

%	File
+4.17%	Mathlib.Data.Tree.RBMap

Import changes for modified files

Dependency changes

File	Base Count	Head Count	Change
Mathlib.Data.Tree.RBMap	48	50	+2 (+4.17%)

Import changes for all files

Files	Import difference
Mathlib.Data.Tree.RBMap	2

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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>

The doc-module for script/declarations_diff.sh contains some details about this script.

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No changes to technical debt.

You can run this locally as

./scripts/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).