[imgui] update to 1.91.8

[rremilian]

Fixes #43564

• 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.

[tomconder]

Don't forget to update the SHA512 hash for the "test-engine" feature in portfile.cmake (line 66).

Old:

• SHA512 4e496075417852a57fed73b34b27a7d4983be9d5a5e95dd854c6c1d3796f96fd43a48166cb07f74e8e50de0ce28c55a4fbf08d2cb4114f1225adc931f9fbae75

New:

• SHA512 c6478efa8d26da7c538bc73c20ff5dfdedee98467128b1d88e5efa384fa8969ff06ff076878c1c4223646ec87ebab4b44081f1cc029fef389eb4c3cfdca57ae7

[tomconder]

:) you will want to run vcpkg x-add-version again to fix the versions database

[rremilian]

@tomconder Thanks ! I missed that one :)