Migrate CI from Travis to GitHub Actions (main branch)

[Copilot]

This PR migrates main from Travis to GitHub Actions using the shared logstash-plugins/.ci@1.x reusable workflows, preserving the effective Travis test scope on this branch. The resolved Travis config (via import) runs unit tests only, so this change adds only the corresponding GHA workflow.

• Scope and parity (main branch)

  • Targets main only (default branch), aligned with the branch-scoped migration issue.
  • Resolved Travis chain:
    • .travis.yml → travis/travis.yml@1.x → defaults.yml, matrix.yml, exec.yml
  • Effective test types detected:
    • Unit tests: yes
    • Integration (INTEGRATION=true): no
    • Secure integration (SECURE_INTEGRATION=true): no
    • Performance (HAS_PERFORMANCE_TESTS=1): no

• Travis → GHA mapping

  Travis matrix/env combination	GHA equivalent
  ELASTIC_STACK_VERSION=9.current DOCKER_ENV=dockerjdk21.env	Unit Tests → tests job (.github/workflows/unit-tests.yml)
  ELASTIC_STACK_VERSION=9.previous DOCKER_ENV=dockerjdk21.env	Unit Tests → tests job
  ELASTIC_STACK_VERSION=8.current DOCKER_ENV=dockerjdk21.env	Unit Tests → tests job
  SNAPSHOT=true ELASTIC_STACK_VERSION=main DOCKER_ENV=dockerjdk21.env	Unit Tests → tests job
  SNAPSHOT=true ELASTIC_STACK_VERSION=9.current DOCKER_ENV=dockerjdk21.env	Unit Tests → tests job
  SNAPSHOT=true ELASTIC_STACK_VERSION=9.previous DOCKER_ENV=dockerjdk21.env	Unit Tests → tests job
  SNAPSHOT=true ELASTIC_STACK_VERSION=8.current DOCKER_ENV=dockerjdk21.env	Unit Tests → tests job
  Conditional performance stage (HAS_PERFORMANCE_TESTS=1)	Not migrated because plugin does not enable it

• Workflow changes

  • Added:
    • .github/workflows/unit-tests.yml
  • Uses reusable workflow:
    • logstash-plugins/.ci/.github/workflows/unit-tests.yml@1.x
  • Includes required caller shape:
    • tests + scheduled jobs
    • secrets: inherit on both jobs
    • scheduled matrix over active branch list ([main])

• README badges

  • Replaced Travis badge with GitHub Actions badge:
    • unit-tests.yml badge, pinned with ?branch=main

• Travis retirement

  • Deleted:
    • .travis.yml

jobs:
  tests:
    uses: logstash-plugins/.ci/.github/workflows/unit-tests.yml@1.x
    secrets: inherit
  scheduled:
    uses: logstash-plugins/.ci/.github/workflows/unit-tests.yml@1.x
    secrets: inherit
    with:
      ref: ${{ matrix.branch }}