[META]: Various maintenance patches

[amotl]

Hi there,

some patches have accumulated while applying regular maintenance work. They address minor vulnerability reports issued by scanners like Grype and Trivy and also adjust or remove some outdated resources.

• Stop pinning the pip package, essentially updating to the most recent 21.3 patch release #200
• Align Dockerfile for nightly builds with Dockerfile for GA builds #201
• Invoke yum upgrade before yum install #202
• Update tests/README. Docker Toolbox is a thing of the past. #203
• Maintenance: Remove early Dockerfile templates for CrateDB 3.1 and 4.1 #204
• CI: Use most recent versions of GitHub Action recipes. Use Python 3.10. #210
• Use AlmaLinux 9 as base image #206
• Raise default heap size? #208

With kind regards,
Andreas.