Auto merge of #28644 - gian-carlo-d:add-ubuntu-21.10-compatibility, r=jdm

Allow builds / tests on Ubuntu 21.10

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This PR updates the version check in `python/servo/bootstrap.py` for Ubuntu to `> 21.10` to accommodate the latest version. It also updates the `clang-format` check in `test_commands.py` to use `find()` instead of `startswith()` to account for possible distro-specific insertions into the `clang-format --version` command.

Ubuntu, for example, outputs `Ubuntu clang-format version 13.0.0-2`, which caused `version.startswith()` to throw an error as it looked for `clang-format` at the beginning of the string. This would cause `./mach test-tidy` to skip the CPP formatting portion.
![image](https://user-images.githubusercontent.com/17208534/146147849-92dd9849-5fc4-4632-b8d0-23b51dc34706.png)

These two changes should help run bootstrap and test commands on the latest Ubuntu version.
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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors

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- [X] These changes do not require tests because: I do not believe there are any current tests for the version checks (as the version checks are in-and-of themselves a test before running the bootstrap/test commands)

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