Migrate CI from Travis to GitHub Actions (main branch)

[Copilot]

This PR migrates CI for the main branch from Travis to GitHub Actions using the shared logstash-plugins/.ci@1.x reusable workflows. The resolved Travis config (.travis.yml + imports) runs unit-test matrix jobs only, so the migration preserves that scope exactly.

• Branch scope and issue alignment

  • Targets main only (default branch), per migration issue scope.
  • Source Travis config was fully resolved via:
    • logstash-plugins/.ci:travis/travis.yml@1.x
    • travis/defaults.yml@1.x
    • travis/matrix.yml@1.x
    • travis/exec.yml@1.x

• CI workflow migration (parity with resolved Travis matrix)

  • Added: .github/workflows/unit-tests.yml
  • Uses reusable workflow: logstash-plugins/.ci/.github/workflows/unit-tests.yml@1.x
  • Includes required trigger/model shape:
    • push/pull_request on main
    • scheduled fan-out matrix (branch: [main]) with ref: ${{ matrix.branch }}
    • workflow_dispatch
  • Includes secrets: inherit on both tests and scheduled jobs.

• Travis matrix/env → GHA mapping

  Resolved Travis entry	Equivalent GHA coverage
  ELASTIC_STACK_VERSION=9.current DOCKER_ENV=dockerjdk21.env	unit-tests.yml reusable matrix/defaults
  ELASTIC_STACK_VERSION=9.previous DOCKER_ENV=dockerjdk21.env	unit-tests.yml reusable matrix/defaults
  ELASTIC_STACK_VERSION=8.current DOCKER_ENV=dockerjdk21.env	unit-tests.yml reusable matrix/defaults
  SNAPSHOT=true ELASTIC_STACK_VERSION=main DOCKER_ENV=dockerjdk21.env	unit-tests.yml reusable matrix/defaults
  SNAPSHOT=true ELASTIC_STACK_VERSION=9.current DOCKER_ENV=dockerjdk21.env	unit-tests.yml reusable matrix/defaults
  SNAPSHOT=true ELASTIC_STACK_VERSION=9.previous DOCKER_ENV=dockerjdk21.env	unit-tests.yml reusable matrix/defaults
  SNAPSHOT=true ELASTIC_STACK_VERSION=8.current DOCKER_ENV=dockerjdk21.env	unit-tests.yml reusable matrix/defaults
  Performance stage gated by HAS_PERFORMANCE_TESTS=1	Not added (no plugin-level enablement in this repo)
  INTEGRATION=true / SECURE_INTEGRATION=true entries	Not present in resolved config; no integration workflows added

• Documentation badge update

  • Replaced Travis badge in README.md with:
    • Unit Tests GHA badge pointing to actions/workflows/unit-tests.yml
    • badge URL scoped with ?branch=main

• Travis retirement

  • Deleted .travis.yml.
  • No additional Travis-only directories/files were present.

• Representative workflow snippet

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