stable charm consumes ppa:landscape/self-hosted-*beta*

[Nobuto Murata]

This happened again. In addition to fixing it, could we have a guardrail for this not to happen again?

The previous bug report:
https://bugs.launchpad.net/landscape-charm/+bug/2015212

$ juju info landscape-server
...
channels: |
  latest/stable: 107 2024-02-15 (107) 7MB amd64, arm64 ubuntu@22.04, ubuntu@20.04
  latest/candidate: ↑
  latest/beta: 107 2024-02-15 (107) 7MB amd64, arm64 ubuntu@22.04, ubuntu@20.04
  latest/edge: 108 2024-02-23 (108) 7MB amd64, arm64 ubuntu@22.04, ubuntu@20.04

$ juju download landscape-server --base ubuntu@22.04
Fetching charm "landscape-server" revision 107 using "stable" channel and base "amd64/ubuntu/22.04"
Install the "landscape-server" charm with:
    juju deploy ./landscape-server_r107.charm

$ unzip -d landscape-server ./landscape-server_r107.charm

$ grep -A3 landscape_ppa: landscape-server/config.yaml
  landscape_ppa:
    type: string
    default: "ppa:landscape/self-hosted-beta"
    description: The PPA from which Landscape Server will be installed.