Respond to the retirement of single ISSUE_TEMPLATE.md

[jennybc]

Brought up in #2068 (comment)

The legacy ISSUE_TEMPLATE.md feature will be retired on March 30, 2025. As a replacement, we encourage creating an ISSUE_TEMPLATE/ subdirectory in any of the supported folders to store multiple issue templates. You can then use the template query parameter to specify which template should populate the issue body. For more details, see the documentation.

After March 30, 2025, repositories still using ISSUE_TEMPLATE.md will default to a blank issue form, allowing users to start fresh when creating issues.

https://github.blog/changelog/2025-02-18-github-issues-projects-february-18th-update/#%f0%9f%8c%85-single-issue-templates-issue_template-md-will-be-retired

A (possibly incomplete) list of things that need to happen:

• Modify (remove?) use_tidy_issue_template()
• Modify (remove?) single issue template files in tidyverse / r-lib repos. Or, rather, plan some attack on that for tidyverse dev day and/or spring cleaning.