Migrate CI from Travis to GitHub Actions (main branch)

[Copilot]

This PR migrates CI on main from Travis to GitHub Actions using logstash-plugins/.ci@1.x, preserving the effective Travis test matrix. The resolved Travis config runs unit-test matrix jobs only, so the migration adds only the unit-test consumer workflow.

• Scope / branch

  • Targets main (default branch).
  • Issue reference: Migrate CI from Travis to GitHub Actions (main branch) #12

• Workflow migration (Travis parity)

  • Added: .github/workflows/unit-tests.yml
  • Uses reusable workflow: logstash-plugins/.ci/.github/workflows/unit-tests.yml@1.x
  • Includes required triggers and structure (push, pull_request, schedule, workflow_dispatch; tests + scheduled; secrets: inherit on both jobs).

• Resolved Travis matrix → GHA mapping

  Resolved Travis env/job	GHA equivalent
  ELASTIC_STACK_VERSION=9.current DOCKER_ENV=dockerjdk21.env	unit-tests.yml → tests (push/PR), scheduled (cron fan-out)
  ELASTIC_STACK_VERSION=9.previous DOCKER_ENV=dockerjdk21.env	unit-tests.yml → tests, scheduled
  ELASTIC_STACK_VERSION=8.current DOCKER_ENV=dockerjdk21.env	unit-tests.yml → tests, scheduled
  SNAPSHOT=true ELASTIC_STACK_VERSION=main DOCKER_ENV=dockerjdk21.env	unit-tests.yml → tests, scheduled
  SNAPSHOT=true ELASTIC_STACK_VERSION=9.current DOCKER_ENV=dockerjdk21.env	unit-tests.yml → tests, scheduled
  SNAPSHOT=true ELASTIC_STACK_VERSION=9.previous DOCKER_ENV=dockerjdk21.env	unit-tests.yml → tests, scheduled
  SNAPSHOT=true ELASTIC_STACK_VERSION=8.current DOCKER_ENV=dockerjdk21.env	unit-tests.yml → tests, scheduled
  HAS_PERFORMANCE_TESTS=1 gated performance stage (not set in this plugin)	No performance workflow added (parity)

• Env var reconciliation from Travis

  • ELASTIC_STACK_VERSION, SNAPSHOT, DOCKER_ENV: re-derived by reusable unit-test workflow matrix.
  • HAS_PERFORMANCE_TESTS: not set by this plugin; remains non-applicable (no performance workflow).
  • No plugin-vendored .ci/ scripts or Travis-provided daemon/service setup to port.

• README badges

  • Replaced Travis badge with GitHub Actions unit-tests badge (?branch=main).

• Travis retirement

  • Deleted .travis.yml.

• Example (new caller workflow pattern)

  jobs:
    tests:
      if: github.event_name != 'schedule'
      uses: logstash-plugins/.ci/.github/workflows/unit-tests.yml@1.x
      secrets: inherit
      with:
        timeout-minutes: 60