CONTRIBUTING.md: remove the transitional "until then, open a blank issue" guidance once the issue forms are on main #4940

Problem

CONTRIBUTING.md, under "How to report a bug or request a feature", says:

So the forms appear in the chooser from the first milestone release that contains them; until then, open a blank issue and give the same details.

That sentence was written for the window in which the issue forms existed on v2/main but not on main. The window closes when #4939 merges (the forms reach main ahead of the v2.0.0 merge, per #4938), and would have closed at the v2.0.0 milestone merge in any case. From then on:

• the forms are in the chooser, so "until then" describes a state that no longer exists, and
• .github/ISSUE_TEMPLATE/config.yml sets blank_issues_enabled: false, so the fallback it recommends ("open a blank issue") is not available.

Copilot raised this on #4939 (#4939 (comment)). It was declined there because that PR must stay byte-identical to v2/main.

Expected

On v2/main, drop or reword the transitional sentence so the paragraph describes the steady state: the chooser is served from the default branch, so a change to the forms goes live at the next milestone merge. The fix then reaches main with the v2.0.0 merge.

The header comments in the three .github/ISSUE_TEMPLATE/*.yml files state that same steady-state rule and need no change.