Disable telemetry in CI and document the opt-out in README (DRUPAL_CMS_TELEMETRY=0)

Problem/Motivation

Drupal CMS sends anonymous install analytics. It skips GitLab CI, GitHub Actions, CircleCI and Tugboat by environment variable, but not local DDEV or Bitbucket Pipelines, so our test rounds of Varbase Starter on DDEV add to the install counts. Our CI and local test builds should not send telemetry. Part of #3633820.

Steps to reproduce

1. Run the Varbase Starter install or varbase-e2e job locally on DDEV.
2. Run ddev drush state:get drupal_cms_telemetry: empty, so telemetry is on.

Proposed resolution

• On 1.0.x, CI: DRUPAL_CMS_TELEMETRY: "0" under variables: in .gitlab-ci.yml, so the install job and the Automated Functional Testing (varbase-e2e) jobs get it. Keep the existing drush state:set drupal_cms_telemetry false after install; the variable covers the install itself.
• Docs: one line in the README on how to opt out.

MR only, no release for this change.

Remaining tasks

• ✅ File an issue
• ❌ Addition/Change/Update/Fix
• ❌ Testing to ensure no regression
• ➖ Automated unit/functional testing coverage
• ❌ Developer Documentation support
• ➖ User Guide Documentation support
• ➖ UX/UI designer responsibilities
• ➖ Accessibility and Readability
• ❌ Reviewed by a human
• ❌ Code review by maintainers
• ❌ Full testing and approval
• ❌ Credit contributors
• ❌ Review with the product owner
• ✅ Update Release Notes
• ✅ Release

User interface changes

• N/A

API changes

• N/A

Data model changes

• N/A

Release notes snippet

• task: #3633828 Turn off Drupal CMS telemetry in CI and document how to opt out in the README.

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