DRIVERS-3427: Finalize client backpressure implementation for phase 1 rollout

[NoahStapp]

This PR updates the client backpressure specification and related specifications to align with the finalized backpressure retry policy.

A summary of these changes with brief reasonings:

• Token buckets are removed entirely. Jitter + exponential backoff is now the sole backpressure retry strategy. Token buckets may be added in the future pending additional analysis and telemetry.
• MAX_RETRIES now defaults to 2 rather than 5. Without token buckets, retry storms occur much more easily. Lowering the default number of maximum retries reduces this risk while still alleviating server overload.
• Server deprioritization (renamed to overload retargeting) is disabled by default. Edge cases with primaryPreferred and secondaryPreferred cause semantic differences that we want users to explicitly opt-in to.
• Two new client options have been added: maxAdaptiveRetries, which configures MAX_RETRIES, and enableOverloadRetargeting, which enables overload retargeting for non-sharded servers. Both are configurable as both client and URI options.
• All tests have been updated to reflect the above, and new tests have been added.

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. Python.
• Test these changes against all server versions and topologies (including standalone, replica set, and sharded
  clusters).

[isabelatkinson]

Rust second implementation: mongodb/mongo-rust-driver#1654

Feedback has been incorporated within the constraints of the design decision.