docs(contributing): drop the transitional blank-issue fallback

[cliffhall]

Closes #4940

Description

Drops the transitional sentence in CONTRIBUTING.md ("How to report a bug or request a feature") that told reporters to open a blank issue until the forms reached main. Once #4939 merges, the forms are in the chooser, and .github/ISSUE_TEMPLATE/config.yml sets blank_issues_enabled: false, so the sentence describes a state that no longer exists and recommends a fallback that is unavailable.

The paragraph now states the steady-state rule, matching the header comments in the three .github/ISSUE_TEMPLATE/*.yml files: a change to the forms goes live at the next milestone merge, not when it merges into v2/main.

Sequencing: once this merges, CONTRIBUTING.md on #4939's branch is refreshed from v2/main, so that PR stays byte-identical to v2/main and what reaches main ahead of v2.0.0 does not carry the stale sentence.

Server Details

• Server: none (repository-wide)
• Changes to: contribution docs

Motivation and Context

Raised by Copilot on #4939 (#4939 (comment)), declined there because that PR must match v2/main. See #4940.

How Has This Been Tested?

No client-observable surface; targeted probes:

1. grep -n -i "blank issue\|until then" CONTRIBUTING.md .github/ISSUE_TEMPLATE/* .github/pull_request_template.md finds only the config.yml comment explaining that blank issues are disabled.
2. npx prettier --check CONTRIBUTING.md passes.
3. The diff is one paragraph in one Markdown file. npm run local:gate was not run: no code, package, workflow or skill is touched.

Breaking Changes

None.

Types of changes

• Documentation update

Checklist

• I have read the MCP Protocol Documentation (not applicable: no protocol surface)
• My changes follows MCP security best practices (not applicable: docs only)
• I have updated the server's README accordingly (not applicable: no server change)
• I have tested this with an LLM client (not applicable: see the probes above)

🤖 Generated with Claude Code

[changeset-bot]

⚠️ No Changeset found

Latest commit: cc21b4e

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[Copilot]

Copilot review overview

🟢 Approval recommended

The focused documentation update matches the issue requirements and existing issue-template guidance.

Review effort: Balanced
Findings: None

What changed in this PR

Updates contribution guidance to reflect the steady-state issue-form workflow after #4939.

Changes:

• Removes the unavailable blank-issue fallback.
• Clarifies when issue-form changes become active.

File	Description
CONTRIBUTING.md	Documents that form changes go live with the next milestone merge.

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