[Repo Assist] Prepare release 8.3.0

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Prepares the RELEASE_NOTES.md for version 8.3.0 — a minor release capturing significant improvements merged since 8.2.0 (February 24).

Changes since 8.2.0

Type	PR	Description
🐛 Bug fix	#432	Fix custom attributes on nested erased types
🐛 Bug fix	#458	Fix GetNestedType on TypeSymbol/ProvidedTypeSymbol for generic provided types
🐛 Bug fix	#459	Fix mutable variable captures in QuotationSimplifier — promote to ref cells
⚡ Performance	#443	Memoize transType in AssemblyCompiler to reduce redundant type translation
⚡ Performance	#457	Cache transTypeRef and transMethRef in assembly compiler
✨ Feature	#428	New warning when all static parameters in a type provider are optional
📚 Docs	#455	Documentation guide overhaul
🧪 Tests	#442	Add coverage tests and Coverage build target
🔧 Toolchain	#431	Update to .NET 8 SDK and toolchain

This is a minor version bump (8.2.0 → 8.3.0) due to the new feature (#428) and significant improvements. If preferred, a patch release (8.2.1) is also reasonable given the emphasis on bug fixes.

Test Status

This PR only modifies RELEASE_NOTES.md. The build/test status for the underlying changes is tracked in the individual PRs listed above (all passed CI before merging).

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To release: merge this PR, then tag v8.3.0 and publish the NuGet package via the existing build pipeline.

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