Salesforce Encrypt module to be deprecated

Salesforce Encrypt module will be deprecated in 8.x-4.0 release.
Sites using this module are encouraged to switch to a JWT OAuth solution, when available.

See #3013414: [meta] Refactor OAuth to allow pluggable authentication for additional background on this effort.

TODO, 8.x-4.0:
* Add deprecation warnings in 8.x-3.x branch
* Update task for 8.x-4.0 that will un-encrypt, purge salesforce_encrypt data, and generally clean up the remnants
* Write documentation on how to get rid of the module included in CR https://www.drupal.org/node/3034230
* Create documentation to create JWT OAuth App in Salesforce, and how to connect it with Drupal https://www.drupal.org/docs/8/modules/salesforce-suite/create-a-oauth-jw...
https://www.drupal.org/docs/8/modules/salesforce-suite/set-up-a-jwt-auth...
* Update change record with links to new docs https://www.drupal.org/node/3034230
* Remove composer requirements for drupal/encrypt and drupal/key