Migrate CI from Travis to GitHub Actions (main branch)

[Copilot]

This PR migrates main from Travis to GitHub Actions with strict parity to the fully-resolved Travis config. The resolved Travis matrix runs unit coverage only, so this migration adds only the unit-test consumer workflow and retires Travis config.

• CI migration (Travis parity)

  • Added .github/workflows/unit-tests.yml calling logstash-plugins/.ci/.github/workflows/unit-tests.yml@1.x.
  • Kept the required branch-aware trigger/schedule shape (push, pull_request, schedule, workflow_dispatch) with scheduled matrix fan-out across 6.x and main.
  • Added workflow token scoping:
    • permissions: contents: read

• Status badge update

  • Replaced the Travis badge in README.md with the GitHub Actions Unit Tests badge pinned to ?branch=main.

• Travis retirement

  • Deleted .travis.yml.

• Resolved Travis → GHA mapping

  • import: logstash-plugins/.ci:travis/travis.yml@1.x (and imports) resolves to base test matrix without INTEGRATION=true, SECURE_INTEGRATION=true, or HAS_PERFORMANCE_TESTS=1.
  • Result: unit workflow added; integration/secure-integration/performance workflows intentionally not added.

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