[gtk] Update to 4.20.3

[SunBlack]

• Changes comply with the maintainer guide.
• SHA512s are updated for each updated download.
• The "supports" clause reflects platforms that may be fixed by this new version.
• Any fixed CI baseline entries are removed from that file.
• Any patches that are no longer applied are deleted from the port's directory.
• The version database is fixed by rerunning ./vcpkg x-add-version --all and committing the result.
• Only one version is added to each modified port's versions file.

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Requires

Dependency librsvg-2.0 found: NO. Found 2.40.20 but need: '>= 2.48'
Run-time dependency librsvg-2.0 found: NO (tried pkgconfig, framework and cmake)
Dependency librsvg-2.0 found: NO. Found 2.40.20 but need: '>= 2.40.23' ; matched: '< 2.41'
Run-time dependency librsvg-2.0 found: NO (tried pkgconfig, framework and cmake)
Looking for a fallback subproject for the dependency librsvg-2.0
ERROR: Subproject librsvg is buildable: NO

[dg0yt]

This is a known blocker.
Updating librsvg is blocked by its use of rust. #48344 would help.