Migrate CI from Travis to GitHub Actions (main branch)

[Copilot]

This PR migrates the main branch CI from Travis to GitHub Actions using the shared logstash-plugins/.ci@1.x reusable workflows, while preserving the test surface defined by the resolved Travis config. For this plugin, resolved Travis matrix coverage is unit-test only, so only the unit workflow is introduced.

• Resolved Travis parity (main)

  • .travis.yml imports logstash-plugins/.ci:travis/travis.yml@1.x (and its defaults/matrix/exec chain).
  • Effective job matrix maps to unit test runs only (no INTEGRATION=true, no SECURE_INTEGRATION=true, no plugin-enabled performance stage).
  • GHA mapping:
    • Travis unit matrix entries (ELASTIC_STACK_VERSION + SNAPSHOT combinations from shared matrix) → .github/workflows/unit-tests.yml (tests + scheduled branch matrix caller).

• GitHub Actions workflow added

  • Added .github/workflows/unit-tests.yml as a caller to:
    • logstash-plugins/.ci/.github/workflows/unit-tests.yml@1.x
  • Includes required trigger/concurrency/secrets/scheduled fanout shape for main.

• CI harness cleanup

  • Removed vendored .ci/setup.sh so bootstrap from shared .ci@1.x provides current assets at runtime instead of retaining stale local script content.

• README badge migration

  • Replaced Travis badge with GitHub Actions unit badge:
    • actions/workflows/unit-tests.yml/badge.svg?branch=main

• Travis retirement

  • Deleted .travis.yml from main.

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