What's New for Tethys 4.4.0

[swainn]

Description

The merge request updates the What's New document for Tethys 4.4.0

Changes Made to Code

• Move Tethys 4.3.0 release notes to prior_releases.rst
• Adds Tethys 4.4.0 release notes to whats_new.rst

Quality Checks

• At least one new test has been written for new code
• New code has 100% test coverage
• Code has been formatted with Black
• Code has been linted with flake8
• Docstrings for new methods have been added
• The documentation has been updated appropriately

[swainn]

If you have contributed to Tethys 4.4.0, please review this PR and add anything that is missing.

[coveralls]

coverage: 100.0%. remained the same
when pulling 1618861 on whats_new_44
into f5759ae on main.

[swainn]

@shawncrawley this is ready for a review