ci: Fix incorrect towncrier configuration

Incorrect towncrier configuration is causing changelog check job
failure. Fixed incorrect towncrier configuration.

Also, added `.gitkeep` to `release_changes` folder, so that git won't
ignore it during a new release when all files under `release_changes`
are deleted.

Signed-off-by: Devaraj Ranganna <devaraj.ranganna@arm.com>