Migrate CI from Travis to GitHub Actions (main branch)

[Copilot]

This PR migrates main branch CI from Travis to GitHub Actions with parity to the fully-resolved Travis config (including imported .ci definitions). It preserves the branch fan-out scheduling pattern required for active branches while retiring Travis config in-repo.

• Scope / Target

  • Targets main branch only.
  • Tracking issue: Migrate CI from Travis to GitHub Actions (main branch) #569

• Travis → GHA parity mapping

  • Resolved Travis config (.travis.yml → .ci@1.x imports) defines the base test matrix only.
  • No INTEGRATION=true matrix entries detected.
  • No SECURE_INTEGRATION=true matrix entries detected.
  • No plugin-enabled performance signal (HAS_PERFORMANCE_TESTS=1) detected.
  • Resulting GHA coverage added: Unit Tests workflow only.

• Workflow changes

  • Added .github/workflows/unit-tests.yml calling:
    • logstash-plugins/.ci/.github/workflows/unit-tests.yml@1.x
  • Includes required trigger shape (push, pull_request, schedule, workflow_dispatch), scheduled branch matrix (['6.x', main]), ref forwarding in scheduled job, and secrets: inherit on both jobs.
  • Added explicit permissions: contents: read.

• Repository cleanup

  • Deleted .travis.yml (Travis retirement for this branch migration).

• README badges

  • Replaced Travis badge with GitHub Actions Unit Tests badge pinned to ?branch=main.

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

  scheduled:
    strategy:
      matrix:
        branch: ['6.x', main]
    uses: logstash-plugins/.ci/.github/workflows/unit-tests.yml@1.x
    secrets: inherit
    with:
      ref: ${{ matrix.branch }}
      timeout-minutes: 60