docs/contribution-model.md: drop the transitional "until then the chooser offers a blank issue" wording #4962

Problem

docs/contribution-model.md on v2/main, below the "Where the policy is stated" table, says:

GitHub serves the issue forms and config.yml from the default branch (main). Development happens on v2/main, so the forms go live at the first milestone merge that contains them. Until then the chooser offers a blank issue, and the close comments below point at the policy on v2/main, where it already applies.

This is the same transitional wording #4940 removed from CONTRIBUTING.md. The forms reach main with #4939, ahead of the v2.0.0 merge, and config.yml sets blank_issues_enabled: false. So "until then the chooser offers a blank issue" will describe a state that no longer exists, and this file reaches main at v2.0.0 still saying it.

Expected

The paragraph states the steady-state rule, as CONTRIBUTING.md and the issue-form headers do: a change to the forms or config.yml goes live at the next milestone merge, not when it merges into v2/main. The close comments' links to the policy on v2/main stay as they are; that is still where a policy change lands first.