Migrate CI from Travis to GitHub Actions (main branch)

[Copilot]

This PR migrates main from Travis CI to GitHub Actions using the shared logstash-plugins/.ci@1.x reusable workflow, while preserving the resolved Travis unit-test matrix exactly.
Target branch: main • Issue: #95

• What changed

  • Added .github/workflows/unit-tests.yml as the GitHub Actions caller workflow
  • Replaced the Travis badge in README.md with a Unit Tests GitHub Actions badge for main
  • Removed .travis.yml

• Workflow files added

  • .github/workflows/unit-tests.yml

• Resolved Travis → GHA parity

  Travis on this branch imported logstash-plugins/.ci:travis/travis.yml@1.x, which resolves to:

  • shared defaults
  • shared matrix
  • shared exec

  On this repo, that resolves to unit tests only:

  • no INTEGRATION=true
  • no SECURE_INTEGRATION=true
  • no HAS_PERFORMANCE_TESTS=1
  • no repo-local .ci/ overrides
  • no repo-local Travis env/service additions

  Resolved Travis env	GHA mapping
  ELASTIC_STACK_VERSION=9.current DOCKER_ENV=dockerjdk21.env	unit-tests.yml → reusable unit-tests matrix
  ELASTIC_STACK_VERSION=9.previous DOCKER_ENV=dockerjdk21.env	unit-tests.yml → reusable unit-tests matrix
  ELASTIC_STACK_VERSION=8.current DOCKER_ENV=dockerjdk21.env	unit-tests.yml → reusable unit-tests matrix
  SNAPSHOT=true ELASTIC_STACK_VERSION=main DOCKER_ENV=dockerjdk21.env	unit-tests.yml → reusable unit-tests matrix
  SNAPSHOT=true ELASTIC_STACK_VERSION=9.current DOCKER_ENV=dockerjdk21.env	unit-tests.yml → reusable unit-tests matrix
  SNAPSHOT=true ELASTIC_STACK_VERSION=9.previous DOCKER_ENV=dockerjdk21.env	unit-tests.yml → reusable unit-tests matrix
  SNAPSHOT=true ELASTIC_STACK_VERSION=8.current DOCKER_ENV=dockerjdk21.env	unit-tests.yml → reusable unit-tests matrix

• Matrix preservation

  • The caller pins the reusable workflow to the same effective stack-version set Travis ran on this branch: 8.current, 9.previous, 9.current, and main
  • This preserves the Travis shape without introducing extra test combinations

with:
  elastic-stack-versions: '["8.current","9.previous","9.current","main"]'
  timeout-minutes: 60

• README badge update

  • Replaced the Travis badge with:
    • Unit Tests → actions/workflows/unit-tests.yml/badge.svg?branch=main

• Travis removal

  • Deleted .travis.yml
  • No additional Travis-only repo files were present to remove