fix: ListApps now discovers plain-manifests directories (#8949) #22672

Job	Run time
Build & cache Go code	4m 25s
Build, test & lint UI code	3m 7s
Ensure Go modules synchronicity	32s
Check changes to generated code	13m 14s
Lint Go code	5m 5s
Run unit tests with -race for Go packages	12m 29s
Run unit tests for Go packages	6m 20s
Run end-to-end tests (v1.27.2)	47m 58s
Run end-to-end tests (v1.26.0)	48m 1s
Run end-to-end tests (v1.25.4)	52m 4s
Run end-to-end tests (v1.24.3)	55m 0s
Process & analyze test artifacts	0s
	4h 8m 15s

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