feat: get released version from versions.txt to render `README.md` #7327

Job	Run time
build (11)	13m 21s
build(8) except for gapic-generator-java	4m 10s
build (17)	12m 35s
build(21) except self-service clients	11m 36s
build(8) for gapic-generator-java	9m 36s
lint	25s
compatibility	1m 39s
showcase (11)	4m 17s
showcase (17)	4m 27s
showcase-native	8m 28s
showcase-clirr	2m 59s
unmanaged_dependency_check	4m 34s
gapic-generator-java-bom	1m 58s
dependency_analyzer_unit_test	23s
	1h 20m 28s

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