Migrate 6.x CI from Travis to GitHub Actions reusable unit workflow

[Copilot]

This PR migrates CI on the 6.x branch from Travis to GitHub Actions using the shared logstash-plugins/.ci reusable workflows. The change preserves Travis test coverage by carrying forward all resolved Travis test-matrix combinations into the GHA unit workflow path.

• Branch scope and parity target

  • Targets 6.x only.
  • Resolved .travis.yml import chain (travis/travis.yml@1.x → defaults.yml, matrix.yml, exec.yml) and mapped all effective Travis env combinations.

• Workflow migration

  • Added: .github/workflows/unit-tests.yml
  • Uses reusable workflow: logstash-plugins/.ci/.github/workflows/unit-tests.yml@1.x
  • Configured for pull_request on 6.x, plus push and workflow_dispatch.
  • Added workflow token hardening (permissions: contents: read) and 6.x-aware concurrency cancellation behavior.

• Travis → GHA matrix mapping (full coverage)

  Resolved Travis env combination	GHA coverage
  ELASTIC_STACK_VERSION=9.current DOCKER_ENV=dockerjdk21.env	Unit Tests reusable workflow matrix (elastic-stack-version=9.current, snapshot=false)
  ELASTIC_STACK_VERSION=9.previous DOCKER_ENV=dockerjdk21.env	Unit Tests reusable workflow matrix (elastic-stack-version=9.previous, snapshot=false)
  ELASTIC_STACK_VERSION=8.current DOCKER_ENV=dockerjdk21.env	Unit Tests reusable workflow matrix (elastic-stack-version=8.current, snapshot=false)
  SNAPSHOT=true ELASTIC_STACK_VERSION=main DOCKER_ENV=dockerjdk21.env	Unit Tests reusable workflow matrix (elastic-stack-version=main, snapshot=true)
  SNAPSHOT=true ELASTIC_STACK_VERSION=9.current DOCKER_ENV=dockerjdk21.env	Unit Tests reusable workflow matrix (elastic-stack-version=9.current, snapshot=true)
  SNAPSHOT=true ELASTIC_STACK_VERSION=9.previous DOCKER_ENV=dockerjdk21.env	Unit Tests reusable workflow matrix (elastic-stack-version=9.previous, snapshot=true)
  SNAPSHOT=true ELASTIC_STACK_VERSION=8.current DOCKER_ENV=dockerjdk21.env	Unit Tests reusable workflow matrix (elastic-stack-version=8.current, snapshot=true)

  • No INTEGRATION=true, SECURE_INTEGRATION=true, or plugin-local HAS_PERFORMANCE_TESTS=1 was present in resolved Travis config, so no integration/secure/performance workflow files were added.

• README badge migration

  • Replaced Travis badge with Unit Tests GitHub Actions badge scoped to 6.x:
    • actions/workflows/unit-tests.yml/badge.svg?branch=6.x

• Travis retirement

  • Deleted .travis.yml.

jobs:
  tests:
    concurrency:
      group: ${{ github.workflow }}-${{ github.ref }}
      cancel-in-progress: ${{ github.ref != 'refs/heads/6.x' }}
    uses: logstash-plugins/.ci/.github/workflows/unit-tests.yml@1.x
    with:
      timeout-minutes: 60