fix-ci: Add .gitkeep to towncrier ignore

This commit fixes failing pipelines by changing the towncrier
configuration to ignore '.gitkeep' files.
Towncrier is automatically updated to the latest version in the Docker
image.
Previous versions of towncrier automatically ignored `.gitkeep` in the
`release_changes` subdirectory. This is not true anymore.

Signed-off-by: Reuben Cartwright <Reuben.Cartwright@arm.com>