deprecate: .github/issue_template.md blank issues template

[Malix-Labs]

Why:

Closes: #44311

What's being changed (if available, include any code snippets, screenshots, or gifs):

deprecating mentions of .github/issue_template.md including the blank issues template behavior

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing and the changes look good in the review environment.

[github-actions]

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Table of review links

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Source	Review	Production	What Changed
communities/using-templates-to-encourage-useful-issues-and-pull-requests/configuring-issue-templates-for-your-repository.md	fpt ghec ghes@ 3.21 3.20 3.19 3.18 3.17 3.16	fpt ghec ghes@ 3.21 3.20 3.19 3.18 3.17 3.16

Key: fpt: Free, Pro, Team; ghec: GitHub Enterprise Cloud; ghes: GitHub Enterprise Server

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[Copilot]

Pull request overview

This PR updates the issue templates documentation to remove outdated guidance about the deprecated legacy .github/issue_template.md blank-issue template behavior, aligning the article with current GitHub functionality.

Changes:

• Removes the note describing the legacy .github/issue_template.md behavior for blank issues.
• Keeps the template chooser guidance focused on the supported blank_issues_enabled configuration.