[WIP] DRIVERS-3540 add "substring"

[kevinAlbs]

Summary

Add "substring" as a query type for QE.

Background & Motivation

This is a continuation of an effort to mark QE String (formerly Text) support as GA.

Support for "substringPreview" is not removed. Removal of preview is planned in DRIVERS-3548.

strMaxQueryLength limit

The GA "substring" added further parameter limits than "substringPreview":

strMaxQueryLength cannot be greater than 6.

The "substringPreview" tests used strMaxQueryLength=10, triggering an error when switching to "substring":

strMaxQueryLength (10) must be <= 6 for substring query type of field encryptedText

Both "substring" and "substringPreview" tests are updated to strMaxQueryLength=6 for consistency.

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Please complete the following before merging:

• Is the relevant DRIVERS ticket in the PR title?
• Update changelog.
• Test changes in at least one language driver. (Pending)
• Test these changes against all server versions and topologies (including standalone, replica set, and sharded
  clusters). (Pending)