chore: redirect security vulnerabilities to email

[HonkingGoose]

Changes:

• Add Markdown comment to redirect potential security bugs to email instead

Context:

I think it's good to make sure bug reporters know that they should not use the bug report template for security concerns.
A bug reporter might miss the section "Report a security vulnerability" from the New issue part of GitHub.

This PR adds a Markdown comment, but we can also use a checkbox or line like this in the visible text:

- [ ] This is not a security concern, please read the [security policy](https://github.com/renovatebot/renovate/security/policy) for security concerns

Closes #8119.

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

How I've tested my work (please tick one)

I have verified these changes via:

• Code inspection only, or
• Newly added unit tests, or
• Unit tests + ran on a real repository

[renovate-release]

🎉 This PR is included in version 24.10.3 🎉

The release is available on:

• GitHub release
• 24.10.3

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