misc: add `Accept` header to all RpcV2Cbor requests #3162

Job	Run time
downstream	13m 40s
all-platforms (ubuntu-latest)	5m 14s
all-platforms (macos-latest)	4m 46s
all-platforms (windows-latest)	8m 46s
jvm (8)	3m 27s
protocol-tests	8m 41s
jvm (11)	3m 21s
jvm (17)	3m 23s
jvm (21)	8m 6s
	59m 24s

Provide feedback