[flatbuffers] Fix config filename when crosscompiling

[Eisenheim9]

The CMake config file has been renamed from Flatbuffers-config.cmake to flatbuffers-config.cmake since google/flatbuffers#7502. When cross-compiling the flatc target include is appended to the old file, which leads to errors when CMake is trying to find the flatbuffers::flatc target. This PR fixes the config filename.

• Changes comply with the maintainer guide
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• The "supports" clause reflects platforms that may be fixed by this new version
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• 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.

[Eisenheim9]

@microsoft-github-policy-service agree