ci: pin GitHub Actions to server SHAs and add hidden-unicode lint #1551

Job	Run time
checks	1m 43s
tests-with-auth (24.x, $WEAVIATE_137)	5m 20s
tests-without-auth (24.x, $WEAVIATE_136)	6m 15s
tests-without-auth (22.x, $WEAVIATE_137)	5m 24s
tests-without-auth (24.x, $WEAVIATE_134)	5m 13s
tests-without-auth (24.x, $WEAVIATE_135)	5m 23s
tests-without-auth (24.x, $WEAVIATE_137)	5m 37s
tests-without-auth (24.x, $WEAVIATE_133)	5m 22s
publish	-1s
	40m 16s

Provide feedback