mod: bump github.com/google/flatbuffers #14954

Job	Run time
aux	4m 23s
arch (presubmit_arch_linux)	10m 46s
arch (presubmit_arch_freebsd)	6m 34s
arch (presubmit_arch_netbsd)	4m 21s
arch (presubmit_arch_openbsd)	4m 22s
arch (presubmit_arch_darwin)	4m 13s
arch (presubmit_arch_windows)	3m 23s
arch (presubmit_arch_executor)	3m 16s
build	13m 11s
dashboard	3m 29s
gvisor	4m 20s
old	6m 33s
race	8m 47s
race_dashboard	4m 50s
	1h 22m 28s

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