Migrate CI from Travis to GitHub Actions (main branch)

[Copilot]

Replaces Travis CI with GitHub Actions for the main branch by wiring into the shared reusable workflows at logstash-plugins/.ci@1.x.

Analysis

The local .travis.yml is a pure import chain:

.travis.yml → travis/travis.yml@1.x → matrix.yml + defaults.yml + exec.yml

Fully resolved, the matrix runs unit tests only — no INTEGRATION=true, SECURE_INTEGRATION=true, or HAS_PERFORMANCE_TESTS=1 in any entry. No .ci/ scripts are vendored locally, so no harness modernization is needed.

Changes

• Added .github/workflows/unit-tests.yml — calls logstash-plugins/.ci/.github/workflows/unit-tests.yml@1.x
  • Push/PR triggers on both active branches (3.x, main)
  • Daily cron with branch matrix (['3.x', main]) so scheduled runs cover all active branches from the default-branch workflow
  • secrets: inherit on both tests and scheduled jobs (required for Slack failure notifications)
  • Reusable workflow's built-in matrix covers all Travis stack versions (8.current, 9.previous, 9.current, main snapshot variants)
• Updated README.md — swapped Travis badge for GHA Unit Tests badge (?branch=main)
• Deleted .travis.yml

Travis → GHA matrix parity

Travis env	GHA coverage
ELASTIC_STACK_VERSION={9.current,9.previous,8.current}	reusable built-in matrix
SNAPSHOT=true ELASTIC_STACK_VERSION={main,9.current,9.previous,8.current}	reusable built-in matrix

No integration, secure-integration, or performance workflows added — those test types are absent from the resolved Travis matrix.