Update Drupal.gitignore

[zhouhana]

Reasons for making this change:

This template is also valid for Drupal 9. As a matter of fact, when starting new projects you should use Drupal 9. And 8 is only supported until November 2021.

Links to documentation supporting these rule changes:

https://www.drupal.org/project/drupal

[bdougie]

Would it be preferred to remove the version number altogether? https://github.com/github/gitignore#versioned-templates

[zhouhana]

Drupal 7 and earlier versions has different needs for the .gitignore than Drupal 8 and later. So maybe it should say

# gitignore template for Drupal 8 and later

or

# gitignore template for version 8 and later of the Drupal project

or similar.

[bdougie]

gitignore template for Drupal 8 and later
This is preferred. Is it also safe to assume that new Drupal projects will be Drupal 8 or later?

[bdougie]

Linking other Drupal PRs #3386 #3386

[lpalgarvio]

the structure is expected to be roughly the same in 8 vs 9, so the comment change should suffice

but perhaps #3914 should be reworked to consider #3386 and update paths if there are any official changes