Resolve "Test lower bounds of dantro dependencies in extra CI jobs"

This MR adjusts the tox test environments and the CI pipeline to add test jobs that use the lower bounds of the specified dependencies. While the regular test: jobs always use the latest version, the test_minimal_deps: jobs can now assert that the range we specify for all dantro dependencies is actually valid.

The corresponding CI jobs are only run on master, for new tags, and on prepare-release-* branches, in order to use fewer computational resources. These jobs are allowed to fail, such that we can resolve the dependency issue separately.

Can this MR be accepted?

• Implementation ready
  • Where necessary, updated lower bounds for dependencies (h5py, numpy, ruamel.yaml)
  • Corresponding pipeline 11971 succeeds
• No changes to tests necessary
• Documentation updated
  • Release guide and MR templates updated to denote the policy regarding version changes

• Code quality
  • Changes follow coding guidelines
  • Checked code coverage on new and adjusted code

• Ready for merging
  • Pipeline passes without warnings
  • History cleaned-up or squash option set
  • Changelog entry added
  • Version number bumped
  • Reviewed & approved by @herdeanu

Related issues

Closes #137 (closed)