Migrate 6.x CI from Travis to GitHub Actions (unit-tests parity)

[Copilot]

This updates CI on the 6.x branch from Travis to GitHub Actions using the shared reusable workflows in logstash-plugins/.ci@1.x. The migration preserves resolved Travis coverage for this branch and removes Travis artifacts.

• Resolved Travis matrix coverage (source of truth)

  • Local .travis.yml imports logstash-plugins/.ci:travis/travis.yml@1.x (and its defaults/matrix/exec imports).
  • Resolved jobs are the base test matrix (ELASTIC_STACK_VERSION/SNAPSHOT combinations); no INTEGRATION=true, SECURE_INTEGRATION=true, or repo-level HAS_PERFORMANCE_TESTS=1 signal in this branch’s config.

• GitHub Actions workflows added (parity-only)

  • Added .github/workflows/unit-tests.yml (only).
  • Uses logstash-plugins/.ci/.github/workflows/unit-tests.yml@1.x.
  • Triggers: push and pull_request on 6.x, plus workflow_dispatch.
  • No schedule: block on this non-default branch (cron fanout belongs on main).

• README badge migration

  • Replaced Travis badge with branch-scoped GHA badge:
    • unit-tests.yml/badge.svg?branch=6.x

• Travis retirement

  • Deleted .travis.yml.

• Travis → GHA mapping table (6.x)

  Resolved Travis signal	Outcome in this PR
  Base matrix test jobs	.github/workflows/unit-tests.yml
  INTEGRATION=true entries	Not present → no integration workflow added
  SECURE_INTEGRATION=true entries	Not present → no secure integration workflow added
  HAS_PERFORMANCE_TESTS=1 / _performance requirement in repo config	Not present in repo config → no performance workflow added

name: Unit Tests
on:
  push:
    branches: ['6.x']
  pull_request:
    branches: ['6.x']
  workflow_dispatch:

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