Update workflows to 5.5.0 and 5.6.0

[jmid]

The 5.5.0 release is in alpha (3), so there's now

• a separate 5.5 branch and
• trunk has been bumped to version 5.6

This PR thus

• updates the *550*.yml workflows to target the 5.5 branch,
• adds new *560*.yml workflows targeting the trunk branch,
• removes the old *530*.yml workflows to keep the workflow count down,
• updates the README badges accordingly

To avoid too much renaming and keep some workflow continuity, the existing *550-trunk*.yml workflows just keep their names (like the existing 530* and 540* files are also named -trunk*), despite not targeting trunk anymore... 🤷

[jmid]

CI summary: all 51 workflows passed.

I'll merge.

[jmid]

CI summary for merge to main: all 52 workflows passed