Restrict build/test to platforms with official LinuxCNC images and include Raspbian Bullseye with LinuxCNC 2.8 latest

[zarfld]

Related to #90

Update CI pipeline to restrict builds/tests to platforms with official LinuxCNC images and include Raspbian Bullseye with LinuxCNC 2.8-latest.

• README.md:

  • Document supported platforms and LinuxCNC versions.
  • Provide instructions for developers to replicate CI environments locally.

• .github/workflows/ci.yml:

  • Restrict builds/tests to Raspbian Bullseye, Raspbian Bookworm, and Debian latest.
  • Remove unsupported platforms from the CI pipeline.

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