Update version, readme, and software process for schema 1.2.0

[rly]

Prepare for release of hdmf-common-schema 1.2.0

Before merging:

• [x] Update requirements versions as needed
• [x] Update legal file dates and information in Legal.txt, license.txt, README.rst, docs/source/conf.py, and any other locations as needed
• [x] Update README.md as needed
• [x] Update the version string in docs/source/conf.py and common/namespace.yaml (remove "-alpha" suffix)
• [x] Update docs/source/conf.py as needed
• [x] Update release notes (set release date) in docs/source/format_release_notes.rst and any other docs as needed
• [x] Test docs locally (make fulldoc)
• [x] Push changes to this PR and make sure all PRs to be included in this release have been merged
• [x] Point the HDMF submodule to this branch in the HDMF branch corresponding to this schema version and run HDMF tests
• [ ] Check that the readthedocs build for this PR succeeds (build latest to pull the new branch, then activate and build docs for new branch): https://readthedocs.org/projects/hdmf-common-schema/builds/

After merging:

1. Create release on GitHub releases page with release notes
2. Check that the readthedocs "latest" and "stable" builds run and succeed

See https://hdmf-common-schema.readthedocs.io/en/latest/software_process.html for more details.