[Bug 1929078] Increase maxLength for labeled groups to 71

[scholtzan]

https://bugzilla.mozilla.org/show_bug.cgi?id=1929078
This should fix the increase we are seeing in org.everit.json.schema.ValidationException: #/metrics/labeled_boolean/browser.ui.mirror_for_toolbar_widgets/<some long label>: expected maxLength: 61, actual: <some number bigger than 61>

Checklist for reviewer:

• Commits should reference a bug or github issue, if relevant (if a bug is referenced, the pull request should include the bug number in the title)
• If adding a new field, the field should have a description (see Bug 1652842 - Add a blocklist field so the environment schema works with pre- and post-bug-1647225 data #576 for an example)
• If coming from a fork, run integration tests: ./.github/push-to-trigger-integration <username>:<branchname>

For glean changes:

• Update templates/include/glean/CHANGELOG.md

For modifications to schemas in restricted namespaces (see CODEOWNERS):

• Follow the change control procedure

[chutten]

LGTM

[chutten]

Oh, I think we need a CHANGELOG entry here: https://github.com/mozilla-services/mozilla-pipeline-schemas/blob/main/templates/include/glean/CHANGELOG.md