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 in logstash-plugins/.ci@1.x.

Changes

• Added .github/workflows/unit-tests.yml — calls unit-tests.yml@1.x with the standard tests + scheduled job shape:

  • secrets: inherit on both jobs (required for SLACK_BOT_TOKEN forwarding)
  • notify-slack: false
  • Top-level concurrency block (job-level concurrency is not valid on uses: jobs)
  • Scheduled matrix fans out over [main] via ref: ${{ matrix.branch }}
  • No elastic-stack-versions override — uses the reusable's default modern matrix (8.current, 9.previous, 9.current, main + snapshots)

• Updated README.md — replaced Travis badge with GitHub Actions Unit Tests badge (?branch=main)

• Deleted .travis.yml

Travis matrix analysis

The fully-resolved Travis config (travis.yml@1.x → matrix.yml@1.x) runs only unit tests — no INTEGRATION, SECURE_INTEGRATION, or HAS_PERFORMANCE_TESTS. No .ci/ vendored scripts exist in this repo, and no Travis services:/addons: backing daemons were used. Only a single workflow file is required.

• Fixes Migrate CI from Travis to GitHub Actions (main branch) #26