Migrate CI from Travis to GitHub Actions (5.x branch)

[Copilot]

This PR migrates logstash-output-http CI on the 5.x branch from the imported Travis matrix to GitHub Actions reusable workflows, preserving the Travis coverage actually exercised on this branch. It targets 5.x only and maps the resolved Travis config to a single GHA unit-test workflow.

• Scope

  • Targets branch: 5.x
  • Issue: logstash-plugins/logstash-output-http#159

• Resolved Travis parity

  • .travis.yml imported logstash-plugins/.ci:travis/travis.yml@1.x
  • Fully resolved Travis config ran only the base unit-test matrix from shared .ci
  • No INTEGRATION=true
  • No SECURE_INTEGRATION=true
  • No HAS_PERFORMANCE_TESTS=1
  • No plugin-local matrix overrides for ELASTIC_STACK_VERSION or DISTRIBUTION

• Matrix mapping

  Travis entry / env	GHA coverage
  ELASTIC_STACK_VERSION=9.current DOCKER_ENV=dockerjdk21.env	unit-tests.yml reusable matrix
  ELASTIC_STACK_VERSION=9.previous DOCKER_ENV=dockerjdk21.env	unit-tests.yml reusable matrix
  ELASTIC_STACK_VERSION=8.current DOCKER_ENV=dockerjdk21.env	unit-tests.yml reusable matrix
  SNAPSHOT=true ELASTIC_STACK_VERSION=main DOCKER_ENV=dockerjdk21.env	unit-tests.yml reusable matrix
  SNAPSHOT=true ELASTIC_STACK_VERSION=9.current DOCKER_ENV=dockerjdk21.env	unit-tests.yml reusable matrix
  SNAPSHOT=true ELASTIC_STACK_VERSION=9.previous DOCKER_ENV=dockerjdk21.env	unit-tests.yml reusable matrix
  SNAPSHOT=true ELASTIC_STACK_VERSION=8.current DOCKER_ENV=dockerjdk21.env	unit-tests.yml reusable matrix

• Workflow changes

  • Added .github/workflows/unit-tests.yml
  • Uses logstash-plugins/.ci/.github/workflows/unit-tests.yml@1.x
  • Includes:
    • push / pull_request for ['5.x', main]
    • scheduled branch fan-out across ['5.x', main]
    • workflow_dispatch
    • secrets: inherit on both caller jobs
    • timeout-minutes: 60

  jobs:
    tests:
      uses: logstash-plugins/.ci/.github/workflows/unit-tests.yml@1.x
      secrets: inherit
  
    scheduled:
      strategy:
        matrix:
          branch: ['5.x', main]
      uses: logstash-plugins/.ci/.github/workflows/unit-tests.yml@1.x
      secrets: inherit
      with:
        ref: ${{ matrix.branch }}

• .ci/ reconciliation

  • No vendored .ci/ directory is present in this branch
  • No plugin-local CI scripts required modernization or deletion
  • No Travis-only env vars were consumed by local CI harness files

• README

  • Replaced the Travis badge with a branch-specific GitHub Actions badge for unit-tests.yml
  • Badge now reports status for 5.x

• Travis retirement

  • Deleted .travis.yml
  • No additional Travis-only files were present