Bump mkdocs-material from 8.5.11 to 9.0.15 (#2610)

Bump mkdocs-material from 8.5.11 to 9.0.15 (#2610)

* Bump mkdocs-material from 8.5.11 to 9.0.15

Bumps [mkdocs-material](https://github.com/squidfunk/mkdocs-material) from 8.5.11 to 9.0.15.
- [Release notes](https://github.com/squidfunk/mkdocs-material/releases)
- [Changelog](https://github.com/squidfunk/mkdocs-material/blob/master/CHANGELOG)
- [Upgrade guide](https://github.com/squidfunk/mkdocs-material/blob/master/docs/upgrade.md)
- [Commits](https://github.com/squidfunk/mkdocs-material/compare/8.5.11...9.0.15)

---
updated-dependencies:
- dependency-name: mkdocs-material
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>
* Bump `mkdocs` to minimum version required by `mkdocs-material-9.0.15`

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Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Michael Adkins <michael@prefect.io>
Co-authored-by: Tom Christie <tom@tomchristie.com>