Prepare 0.9.0 release

[garrison]

Remaining action items:

• Test all tutorial notebooks on a hardware backend
• Update authors in CITATION.bib
• Update version [sub]string in all files where it appears in the repository
  • .mergify.yml
  • pyproject.toml
  • .github/workflows/docs.yml
• move release notes to 0.9 directory
• merge everything except this PR on the 0.9 milestone
• build docs locally; proofread the release notes
• merge this PR
• tag the release
  1. git checkout main
  2. git pull
  3. git show (verify it's the commit we want to tag)
  4. git tag -a 0.9.0 -m 0.9.0
  5. git show (double/triple check the tag is on the correct branch)
  6. git push origin 0.9.0
• Wait for CI to finish. The release should then be available on GitHub and pypi.
• create the stable/0.9 branch
  1. start with same commit as above checked out
  2. git checkout -b stable/0.9
  3. git show (check before pushing)
  4. git push --set-upstream origin stable/0.9
• get docs to deploy from the new stable branch (should happen automatically at this point after CI runs both on the new stable branch and then on the gh-pages branch)
• move remaining 0.7.4 milestone items to 0.9.1 milestone and close 0.7.4 & 0.9.0 milestones

Previous minor release PR: #555

Closes #603.

[coveralls]

Pull Request Test Coverage Report for Build 10793920327

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 10793817960:	0.0%
Covered Lines:	2408
Relevant Lines:	2408

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 10794473963

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

---

Totals
Change from base Build 10794454286:	0.0%
Covered Lines:	2408
Relevant Lines:	2408

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💛 - Coveralls