chore(experimenter): fix server-side glean

[relud]

Because

• settings.APP_VERSION is None, and glean app_display_version must not be None
• glean application_id must use a - separator not .
• nimbus enrollments must be a list

This commit

• Changes from settings.APP_VERSION to experimenter.base.app_version()
• Changes glean app id from experimenter.backend to experimenter-backend
• Changes nimbus enrollments fallback value from None to []

Fixes #13858

[relud]

these changes have been verified in stage with the help of @akkomar to resolve the issue of glean server-side pings being rejected at schema validation