Migrate main CI from Travis to reusable GitHub Actions workflows

[Copilot]

This updates main to retire Travis while preserving CI parity with the fully-resolved Travis configuration (.travis.yml + imported .ci files). The resulting setup keeps only the test types actually exercised by this plugin on main.

• CI migration (parity-first)

  • Added .github/workflows/unit-tests.yml using logstash-plugins/.ci/.github/workflows/unit-tests.yml@1.x
  • Kept trigger and concurrency semantics aligned to main (pull_request.branches: [main], non-main cancel-in-progress)
  • Set timeout-minutes: 60
  • Added explicit minimal workflow permissions (contents: read)

• Removed Travis

  • Deleted .travis.yml from the repository

• README status badges

  • Replaced Travis badge with GitHub Actions Unit Tests badge scoped to main (?branch=main)

• Resolved Travis → GHA mapping

  • Base Travis matrix jobs → Unit Tests workflow added
  • INTEGRATION=true entries → none in resolved config
  • SECURE_INTEGRATION=true entries → none in resolved config
  • Performance (HAS_PERFORMANCE_TESTS=1 / active performance jobs) → not enabled for this plugin

jobs:
  tests:
    uses: logstash-plugins/.ci/.github/workflows/unit-tests.yml@1.x
    with:
      timeout-minutes: 60