docs: github: streamline environment in issue templates

[kmk3]

Changes:

• Sync bug_report.md with build_issue.md (reword items and add Linux kernel item)
• Add a colon to the end of every item (to clarify where to add the information)
• Add the Environment section to feature_request.md

The last item is intended as a basic sanity check, as users using an outdated version of firejail may request something that was already implemented (for example, see #6461).

Relates to #4515 #6423.