Allow using `[MustBeVariant]` in generic typed attributes #20398

Job	Run time
📊 Static checks / Code style, file formatting, and docs	2m 35s
🍎 macOS / Editor (target=editor, tests=yes)	5m 26s
🐧 Linux / Editor w/ Mono (target=editor)	5m 19s
🍎 macOS / Template (target=template_release)	3m 14s
🐧 Linux / Editor with doubles and GCC sanitizers (target=editor, tests=yes, dev_build=yes, scu_build=yes, precision=double, use_asan=yes, use_ubsan=yes, linker=gold)	15m 57s
🏁 Windows / Editor (target=editor, tests=yes)	8m 11s
🌐 Web / Template w/ threads (target=template_release, threads=yes)	1m 40s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	6m 15s
🏁 Windows / Template (target=template_release)	8m 1s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	6m 48s
🌐 Web / Template w/o threads (target=template_release, threads=no)	1m 27s
🐧 Linux / Template w/ Mono (target=template_release)	2m 8s
🤖 Android / Template (target=template_release)	3m 58s
🐧 Linux / Minimal template (target=template_release, everything disabled)	1m 14s
🍏 iOS / Template (target=template_release)	2m 40s
🪲 Godot CPP / Build and test Godot CPP	6m 48s
	1h 21m 41s

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