build(client): Add release notes section for legacy API changes (micr…

…osoft#23003)

Since we'll be releasing a lot of legacy API changes and removals in
2.10, I think a dedicated section in the release notes will be useful.
Incidentally this is why the sections were designed to be configurable.

.changeset/rude-views-bake.md

@@ -4,7 +4,7 @@
 "@fluidframework/sequence": minor
 ---
 ---
-"section": deprecation
+"section": legacy
 ---
 
 MergeTree `Client` Legacy API Removed

fluidBuild.config.cjs

@@ -631,9 +631,10 @@ module.exports = {
 	releaseNotes: {
 		sections: {
 			feature: { heading: "✨ New Features" },
-			tree: { heading: "🌳 SharedTree DDS changes" },
+			tree: { heading: "🌳 SharedTree DDS Changes" },
 			fix: { heading: "🐛 Bug Fixes" },
 			deprecation: { heading: "⚠️ Deprecations" },
+			legacy: { heading: "Legacy API Changes" },
 			other: { heading: "Other Changes" },
 		},
 	},