Prepare unittests to be extended with different bazel versions

[furtib]

Why:
We want to support multiple Bazel versions simultaneously.
We should run our test suite on all supported Bazel versions.

What:
Added a matrix strategy to both of the unittest jobs. .github/bazel_version.json contains a list of bazel versions to be used during testing.

Addresses:
#108

[furtib]

Currently, the bazel versions are declared for each job separately.
For this, I have two solutions:

A) Load versions from a file

To have the versions loaded from the same source, I would need to introduce a separate setup job.
Including such a setup job would allow us to load versions from a separate file, but would significantly complicate the workflow file.
Loading versions from a separate file would allow us to load these versions in foss.yaml too. This would require 2 setup jobs, 1 for each YAML file.
To see an example for this see: #72

B) Combine all jobs into 1 large, meta job

Alternatively, we could merge the two jobs and have the matrix strategy also generate the separate OS dynamically.
In this case, the Ubuntu version would be required to use a Docker container too. Sadly, this would mean the version numbers aren't shared between the FOSS and unittests.
This would look like this:

jobs:
  unified_test:
    name: "${{ matrix.config.label }} / Bazel ${{ matrix.bazel_version }}"
    runs-on: ubuntu-24.04
    
    container:
      image: ${{ matrix.config.image }}

    strategy:
      fail-fast: false
      matrix:
        bazel_version: ["6.5.0", "7.0.0"]
        
        config:
          - label: "Ubuntu"
            image: "ubuntu:24.04"
            setup_dir: "ubuntu"
          - label: "RHEL9"
            image: "redhat/ubi9:latest"
            setup_dir: "rhel9"

    steps:
      - name: Checkout repository
        uses: actions/checkout@v4

      - name: Set bazel version to ${{ matrix.bazel_version }}
        run: echo "${{ matrix.bazel_version }}" > .bazelversion
        shell: bash

      - name: Setup environment
        uses: ./.github/platform_environment_setup/${{ matrix.config.setup_dir }}

C) Use alternative solutions:

• Create separate CI files for all bazel versions
• Use the Orchestrator pattern (Create a separate workflow, and start the other workflows from there, setting environment variables)

[furtib]

I have gone ahead with option A

[Szelethus]

This looks like a very elegant solution! LGTM!