fix(NODE-5573): fix saslprep import

[durran]

Description

Require @mongodb-js/saslprep not saslprep

What is changing?

Fixes the saslprep import.

Is there new documentation needed for these changes?

None

What is the motivation for this change?

NODE-5573

Release Highlight

Import of saslprep updated to correct library.

Fixes the import of saslprep to be the correct @mongodb-js/saslprep library.

Double check the following

• Ran npm run check:lint script
• Self-review completed using the steps outlined here
• PR title follows the correct format: type(NODE-xxxx)[!]: description
  • Example: feat(NODE-1234)!: rewriting everything in coffeescript
• Changes are covered by tests
• New TODOs have a related JIRA ticket

[addaleax]

The title should refer to NODE-5573, not NODE-5572, right?