[Task]: Bump threshold for locales completion to 80%

[diox]

Description

We implemented a cron that warns us about locales falling below a certain threshold of completion.

That initial threshold, as per plan to streamline locales on AMO was set to 40%. The next step, once locale communities have been warned, is to set it to 70-80%.

https://github.com/mozilla/addons/issues/15433#issuecomment-2717570957 has the details on the impact it would have if this was enforced today.

Acceptance Criteria

  ### Milestones/checkpoints
  - [ ] Threshold for locale completion is set to 70% or 80%.

Checks

• [x] If I have identified that the work is specific to a repository, I have removed "repository:addons-server" or "repository:addons-frontend"

┆Issue is synchronized with this Jira Task