Fix CKEditor versions

This is all a bit messy (I know, SA-pressure...), but as far as I can tell:

- This issue is a follow-up from: #3215916: Update ckeditor to 4.16.1, where CKEditor was bumped to 4.17
- That happened for 9.2.x and 9.3.x, but not for 9.1.x.
The latter being the reason the patch won't apply to 9.1.x, since CKEditor 4.16.1 is already/still in there after SA-CORE-2021-003.
- The test-fail on 9.3.x seemed like a random one, ordered retest. As soon as that comes back green, I'm going to RTBC

Removed 9.1.x from IS

I stand corrected 9.1.9 was shipped with CKEditor 4.17 (See https://git.drupalcode.org/project/drupal/-/blob/9.1.9/core/assets/vendo...), so 9.1.x should be patched.

By now, I'm confused about why we're going backwards in CKEditor versions, leaving dev-branches using older stuff than actual releases, so I'll re-add 9.1.x to the IS and leave this issue alone.

At least it seemed I was right about something, the 9.3.x fail has been previously logged as random: #3203712: [random test failure] EntityDisplayTest::testExtraFields()

I'm confused about why we're going backwards in CKEditor versions

The SA intended to update to 4.16.1, but unintentionally pulled in additional commits ahead of that tag. 4.17.0 has not yet been released, so going to 4.16.1 is in order to get onto a tagged release.

Thanks @effulgentsia for clearing that up. :)

I've build the Drupal version of CKEditor 4.16.1 locally according to the instructions in core/assets/vendor/ckeditor/build-config.js and got the same result as 3215929.patch, except for the expected cache-buster query string.

So patch 3215929.patch is RTBC for me on 9.3.x and 9.2.x.

To prevent 9.1.x holding up those branches, I've opened follow-up #3215970: [9.1.x] Fix CKEditor versions for that branch

Committed/pushed to 9.3.x and cherry-picked to 9.2.x, thanks!