issue templates: remove preview_only attribute

[timdeschryver]

Why:

Closes [issue link]

Hi, first of all thank you for the template feature, it's a great improvement!

When I tried it out earlier, I encountered a problem with the examples that include a preview_only attribute. It seems like that attribute doesn't exist, resulting in a badly formatted template when we copy paste the snippet.

I also couldn't find more information about the property, for example in https://docs.github.com/en/communities/using-templates-to-encourage-useful-issues-and-pull-requests/syntax-for-githubs-form-schema, so I assume that this property can be removed from the examples.

Here, you can see an example, in which I added the preview_only attribute to our template https://github.com/ngrx/platform/blob/a812382d4a0b60c97f5141053a31d5880c0159c2/.github/ISSUE_TEMPLATE/bug-report.yml

What's being changed:

The preview_only attribute is removed from the examples.

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Content directory changes

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Source	Preview	Production	What Changed
communities/using-templates-to-encourage-useful-issues-and-pull-requests/common-validation-errors-when-creating-issue-forms.md	fpt ghec	fpt ghec

[ramyaparimi]

@timdeschryver
Thanks so much for opening a PR! I'll get this triaged for review ⚡

[janiceilene]

Thanks so much for opening this PR @timdeschryver! I checked with the team and it sounds like preview_only was removed a while back, but the change never made it to the docs 😅 so we really appreciate you catching that 💖

I'll update the branch and get this merged soon ⚡

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