Fix CI errors - slash in branch name and Python version

[georgemccabe]

These changes are needed to fix some deficiencies discovered by pull request #1982 that was auto generated by dependabot.

Change Summary

• Replace / with _ to prevent errors creating docker tags for tests
• Update version of Python used in GitHub Actions workflow files because 3.6 appears to no longer be supported

Note: There is no branch associated with these changes and I did not assign a milestone because these changes are internal and it is not necessary to include it in the bugfix release notes.

Pull Request Testing

• Describe testing already performed for these changes:
  

Named branch with forward slash to test that it properly triggers the testing workflow

• Recommend testing for the reviewer(s) to perform, including the location of input datasets, and any additional instructions:
  

Ensure all automated tests pass

• Do these changes include sufficient documentation updates, ensuring that no errors or warnings exist in the build of the documentation? [Yes]

• Do these changes include sufficient testing updates? [Yes]

• Will this PR result in changes to the test suite? [No]
  

• Please complete this pull request review by 12/9/2022.
  

Pull Request Checklist

See the METplus Workflow for details.

• Add any new Python packages to the METplus Components Python Requirements table.
• Review the source issue metadata (required labels, projects, and milestone).
• Complete the PR definition above.
• Ensure the PR title matches the feature or bugfix branch name.
• Define the PR metadata, as permissions allow.
  Select: Reviewer(s)
  Select: Organization level software support Project or Repository level development cycle Project
  Select: Milestone as the version that will include these changes
• After submitting the PR, select Development with the original issue number.
• After the PR is approved, merge your changes. If permissions do not allow this, request that the reviewer do the merge.
• Close the linked issue and delete your feature or bugfix branch from GitHub.

[JohnHalleyGotway]

I approve of these changes. I see that all tests passed. Inspecting the code, I see the changes described: switching embedding '/' to '_' and updating from python 3.6 to 3.8.