PHPLIB-1680: Remove pre-4.2 wire version checks and test code

[jmikola]

https://jira.mongodb.org/browse/PHPLIB-1680

Supersedes #1695

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.70%. Comparing base (75fa63e) to head (e3ffa90).
⚠️ Report is 5 commits behind head on v2.x.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@             Coverage Diff              @@
##               v2.x    #1801      +/-   ##
============================================
- Coverage     87.72%   87.70%   -0.02%     
+ Complexity     3195     3185      -10     
============================================
  Files           424      424              
  Lines          6296     6289       -7     
============================================
- Hits           5523     5516       -7     
  Misses          773      773              

Flag	Coverage Δ
6.0-replica_set	85.72% <100.00%> (+0.01%)	⬆️
6.0-server	81.71% <100.00%> (+0.01%)	⬆️
6.0-sharded_cluster	?
8.0-replica_set	87.66% <100.00%> (+0.01%)	⬆️
8.0-server	82.55% <100.00%> (+0.01%)	⬆️
8.0-sharded_cluster	?

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