Docs: updating_drupal.md improvements

[ysuarez]

Purpose / why

I wanted to update the composer syntax for updating Drupal, since the current code seemed to be in the style of the older Islandora 8 before the release of Islandora 2.

This is tied to this issue I had created a while ago...
[DOCS] The "Updating Drupal" section may need updating for Drupal 9

What changes were made?

I simplified the composer update syntax, and still kept the older Islandora 8 syntax. I also added a link to the existing page that explains how to update Drupal for ISLE users.

Verification

Those that are familiar with Drupal updates with composer should review my syntax.

Interested Parties

• @Islandora/documentation
• @Islandora/committers

Checklist

Pull-request Reviewer

Pull-request reviewer should ensure the following:

• Does this PR link to related issues?
• Does the proposed documentation align with the Islandora Documentation Style Guide?
• Are the changes accurate, useful, free of typos, etc?

Person Merging

The person merging should ensure the following:

• Does mkdocs still build successfully? (This is indicated by TravisCI passing. To test locally, and see warnings, see How To Build Documentation.)
• If pages are renamed or removed, have all internal links to those pages been fixed?
• If pages are added, have they been linked to or placed in the menu?
• Did the PR receive at least one approval from a committer, and all issues raised have been addressed?