Autogen: Revert "AUTO*_EXECUTABLE: add support for per-config values"

Changing the timestamp file to timestamp_$<CONFIG> causes some user projects to break when using Qt versions older than 6.6.

Revert commits e454314d and fddd0f04 from !8564 (merged) pending further investigation.

Issue: #20074 (closed)
Backport: release
Topic-rename: revert-autogen-exe-per-config