Fix version check

[zrax]

Previously, Vim 8.1 would pass the version check, but fail later in the script due to attempting to use vim 9.0 features.

[github-actions]

At the moment we are not accepting contributions to the repository.

Feedback for Copilot.vim can be given in the Copilot community discussions.

[tpope]

How is it failing, exactly? The design is deliberate; users on 8.1 still get a :Copilot command to tell them the version is too old.

[zrax]

This is what I see from 8.1.1847 (Ubuntu 20.04) at startup, without the change:

Error detected while processing /home/zrax/.vim/pack/github/start/copilot.vim/plugin/copilot.vim:
line   56:
E216: No such group or event: InsertLeavePre       * call s:Event('InsertLeavePre')
Press ENTER or type command to continue

[tpope]

Thanks, it wasn't reproducible on 8.1.0000. I'll fix it.