Remove Subversion from the um and um_createbc Spack package recipes

[penguian]

This issue primarily affects spack_repo/access/nri/build_systems/um_base.py but also affects the um and um_createbc Spack package recipes (SPRs) themselves.

Remove the ability to build the um and um_createbc Spack packages using the MOSRS Subversion mirror on Gadi.

Changes would include (double check these):

In um_base.py:

• Use the GitHub URL for the um source code.
• Remove _revision.
• In version , use Git tags for UM versions 13.0 to 13.9.
• Add versions with Git tags for the supported models.
• Remove "um" from _projects, since it no longer needs to use _dynamic_resource.
  See https://spack.readthedocs.io/en/v1.1.1/spec_syntax.html#git-versions which says that even when a Git ref is used in the spec in place of a Spack version, that "Spack always needs to associate a Spack version with the git reference, which is used for version comparison. This Spack version is heuristically taken from the closest valid git tag among the ancestors of the git ref." Therefore um_ref is no longer needed.
• Remove _rev_variants and _sources_variants.
• Remove "location_var" and "sources_var" from _project_cfg.
• Remove github_models, since all models use GitHub.
• Explicitly set the following environment variables to the empty string, without a warning:
  config_revision, as well as {project}_project_location, {project}_rev, {project}_sources (for "um" and all projects in projects_needed).
• Remove fetcher as it is no longer needed.
• Set the config_root_path environment variable to the contents of self.stage.source_path, which is where the UM source code Git repo is cloned to.
• In build(), set extract.location[um] to the contents of self.stage.source_path.