issues: make proprietary driver comparison optional

[Rohithmatham12]

Summary

• Replace the required proprietary-driver confirmation checkbox in the functional bug template with an optional comparison field.
• Keep the comparison request for cases where the reporter can test the same driver version with a proprietary kernel module.
• Let reporters explain when that comparison is unavailable for their GPU, driver branch, or packaging setup.

Fixes #1147.

Testing

• git diff --check

This only changes the GitHub issue template.

[CLAassistant]

All committers have signed the CLA.