Add MuteSecurityFindings endpoint spec #17627

Job	Run time
test / test (jruby-9.4.12.0, ubuntu-latest)	3m 33s
pre-commit / pre-commit	1m 12s
test / test (3.2, ubuntu-latest)	3m 1s
examples / examples	2m 46s
test / test (2.7, ubuntu-latest)	2m 57s
report	12s
	13m 41s

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