Migrate main CI from Travis to GitHub Actions reusable unit workflow

[Copilot]

This migrates the plugin’s main branch CI from Travis to GitHub Actions using the shared logstash-plugins/.ci@1.x reusable workflows. The resolved Travis config for this repo only exercised the base unit-test matrix, so the migration adds only unit-test coverage in GHA.

• Workflow migration (Travis → GHA)

  • Added .github/workflows/unit-tests.yml that calls logstash-plugins/.ci/.github/workflows/unit-tests.yml@1.x.
  • Preserved required trigger shape (push/pull_request on main, workflow_dispatch, and scheduled matrix fan-out).
  • Added secrets: inherit and notify-slack: false on both normal and scheduled jobs.

• Security hardening on workflow caller

  • Added explicit least-privilege token scope:
    • permissions: contents: read

• Repository cleanup

  • Removed legacy .travis.yml.
  • Replaced Travis badge in README.md with the unit-tests.yml GitHub Actions badge scoped to main.

• Resolved Travis matrix parity

  • .travis.yml imported logstash-plugins/.ci:travis/travis.yml@1.x (and transitively defaults.yml, matrix.yml, exec.yml).
  • No plugin-local INTEGRATION=true, SECURE_INTEGRATION=true, or HAS_PERFORMANCE_TESTS=1 entries were present; therefore no integration/secure/performance workflow files were introduced.

jobs:
  tests:
    uses: logstash-plugins/.ci/.github/workflows/unit-tests.yml@1.x
    secrets: inherit
    with:
      notify-slack: false
      timeout-minutes: 60

• Fixes Migrate CI from Travis to GitHub Actions (main branch) #10

[donoghuc]

pre-existing failures. Clean migration. Merging.