Docs: add code fences to the issue template

[not-an-aardvark]

What is the purpose of this pull request? (put an "X" next to item)

[x] Documentation update
[ ] Bug fix (template)
[ ] New rule (template)
[ ] Changes an existing rule (template)
[ ] Add autofixing to a rule
[ ] Add a CLI option
[ ] Add something to the core
[ ] Other, please explain:

Please check each item to ensure your pull request is ready:

• I've read the pull request guide
• (n/a) I've included tests for my change
• I've updated documentation for my change (if appropriate)

What changes did you make? (Give an overview)

There have been a few cases (e.g. #7164 and #7251) where people have pasted source code into the issue template without putting it in a code block. This can make it confusing to look at the issue, because markdown hides all indentation and HTML-like tags.

This PR adds a code block to the issue template to encourage people to put their source code/configs in code blocks.

Is there anything you'd like reviewers to focus on?

Nothing in particular, but I'm certainly open to bikeshedding on the wording/formatting.

[mention-bot]

@not-an-aardvark, thanks for your PR! By analyzing the annotation information on this pull request, we identified @kaicataldo, @nzakas and @platinumazure to be potential reviewers

[eslintbot]

LGTM

[vitorbal]

LGTM. Thanks!