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

[Copilot]

This PR migrates CI for the 5.x branch from Travis to GitHub Actions using the shared reusable workflows in logstash-plugins/.ci@1.x, and removes Travis configuration from this branch. It keeps branch coverage semantics for 5.x + main and updates repository status badges accordingly.

• Scope / branch targeting

  • Targets 5.x only (non-default branch), aligned with the migration issue scope: Migrate CI from Travis to GitHub Actions (5.x branch) #154

• Workflow migration (Travis → GHA)

  • Added: .github/workflows/unit-tests.yml
  • Uses reusable workflow: logstash-plugins/.ci/.github/workflows/unit-tests.yml@1.x
  • Includes:
    • push/pull_request on ['5.x', main]
    • daily schedule fanout matrix over ['5.x', main] with ref: ${{ matrix.branch }}
    • workflow_dispatch
    • secrets: inherit on both caller jobs
    • per-branch concurrency cancellation behavior matching the shared migration pattern
    • explicit permissions: contents: read

• Travis parity mapping (resolved config → GHA caller)

  • Travis base matrix + plugin overrides resolve to unit-test executions; migrated to Unit Tests workflow caller.
  • Mapping:

  Resolved Travis env/job	GHA equivalent
  ELASTIC_STACK_VERSION=8.current	unit-tests.yml → tests job (reusable unit-tests matrix)
  ELASTIC_STACK_VERSION=9.previous	unit-tests.yml → tests job
  ELASTIC_STACK_VERSION=9.current	unit-tests.yml → tests job
  SNAPSHOT=true ELASTIC_STACK_VERSION=8.current	unit-tests.yml → tests job
  SNAPSHOT=true ELASTIC_STACK_VERSION=9.previous	unit-tests.yml → tests job
  SNAPSHOT=true ELASTIC_STACK_VERSION=9.current	unit-tests.yml → tests job
  SNAPSHOT=true ELASTIC_STACK_VERSION=main	unit-tests.yml → tests job
  Scheduled coverage for active branches	unit-tests.yml → scheduled matrix job (branch: ['5.x', main])

• README badges

  • Replaced Travis badge with branch-specific GitHub Actions badge:
    • Unit Tests badge URL now points to actions/workflows/unit-tests.yml/badge.svg?branch=5.x

• Travis removal

  • Deleted .travis.yml from 5.x.

• Example snippet (new caller pattern)

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