Migrate CI from Travis to GitHub Actions (6.x branch)

[Copilot]

This migrates the 6.x branch off Travis while preserving the resolved Travis test matrix. For this branch, the imported Travis config only exercised the base/unit matrix, so the migration adds the matching GitHub Actions workflow and removes Travis-specific configuration.

• CI migration

  • Added .github/workflows/unit-tests.yml as a consumer workflow for logstash-plugins/.ci/.github/workflows/unit-tests.yml@1.x
  • Kept the required branch coverage for both active branches and the scheduled branch matrix:
    • push / PR on 6.x and main
    • scheduled fan-out over ['6.x', 'main']
  • Forwarded secrets with secrets: inherit
  • Set explicit permissions: contents: read

• Travis parity

  • Mapped the resolved Travis matrix to the reusable unit-test workflow only
  • Did not add integration, secure-integration, or performance workflows because those signals do not appear in the resolved Travis config for this branch

• Repository cleanup

  • Removed .travis.yml
  • Replaced the Travis badge in README.md with the 6.x GitHub Actions unit-test badge

• Vendored CI scripts

  • Kept the existing .ci/ scripts in place; they contain plugin-specific behavior and did not require Compose v2 modernization

Example of the new workflow shape:

jobs:
  tests:
    if: github.event_name != 'schedule'
    uses: logstash-plugins/.ci/.github/workflows/unit-tests.yml@1.x
    secrets: inherit
    concurrency:
      group: ${{ github.workflow }}-${{ github.ref }}
      cancel-in-progress: ${{ github.ref != 'refs/heads/6.x' && github.ref != 'refs/heads/main' }}
    with:
      timeout-minutes: 60

  scheduled:
    if: github.event_name == 'schedule'
    strategy:
      matrix:
        branch: ['6.x', 'main']
    uses: logstash-plugins/.ci/.github/workflows/unit-tests.yml@1.x
    secrets: inherit
    with:
      ref: ${{ matrix.branch }}
      timeout-minutes: 60