Update technical note for Object Cache with Drupal

[ccharlton]

D9 and D7 needed the steps that are no longer considered optional.

Closes #

Summary

Object Cache for Drupal - changes optional to recommended and makes sure D9 people see the info.

• [ ] Other prerequisites that must be completed before merging this PR

Release:

• [X] When ready
• [ ] After date: $DATE

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Post Launch

Do not remove - To be completed by the docs team upon merge:

• [ ] Redirect /old-path/ => /new-path/ (if applicable)
• [ ] Include/exclude pages ^ respectively within docs search service provider (if applicable)