Relax `ProtocolType` requirement on `ProtocolObject` #2371

Job	Run time
Check MSRV	1m 11s
Check formatting	9s
Lint macOS 64bit	2m 31s
Lint iOS 32bit	52s
Lint GNUStep + exceptions	1m 16s
Lint header-translator	28s
Lint test-assembly	27s
Test macOS 12	4m 24s
Assembly tests	52s
Compiler UI	1m 24s
Verify header translator output	1m 6s
Check icrate features	0s
${{ matrix.name }}	0s
Test Compiler-RT	30s
Test GNUStep with libobjc2 v1.9	1m 40s
Test GNUStep with libobjc2 v2.0	1m 31s
Test on iOS simulator w. dinghy	0s
Test GNUStep with libobjc2 v2.1 on nightly	1m 47s
Test GNUStep 32bit	1m 39s
	21m 47s

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