[CI] rolling is now the default branch, update CI and README accordingly

[mikaelarguedas]

Also bumps runner to ubuntu-latest but doesnt matter as everything runs in containers

This is an attempt to revive CI that has been off for a long time

[mikaelarguedas]

and nightly build looks good to me, but we do not have any CI for iron and humble? i did not really know the history for sros2 CI...

my feeling is that this workflow has 2 main goals ->

• test_latest: generate coverage on pull requests and merges
• test_nightly: catch any regression overnight

for released distros I believe we can keep relying on the PR jobs of the ROS buildfarm :+1:

[mikaelarguedas]

I'll get this in as-is and work on gettign latest back to working in a separate PR