Extend RC testing period

Update the release schedule to extend the RC testing period to have
enough time to run all the planned tests and still have some buffer
to release and a new RC in case of a major issue. With this new
schedule, RC testing period will be two weeks.

This new release schedule does not affect the release date, which
means that RC testing periods will overlap with different releases.

Signed-off-by: Walter Lozano <walter.lozano@collabora.com>