Fixes an availability issue for DiscardingTaskGroup on watchOS

[lovetodream]

DiscardingTaskGroup is only available on watchOS >=10.0 :).

This patch allows the ConnectionPoolModule to be successfully built for watchOS.

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (e60e495) 59.81% compared to head (03bfb62) 59.84%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #448      +/-   ##
==========================================
+ Coverage   59.81%   59.84%   +0.02%     
==========================================
  Files         124      124              
  Lines        9915     9937      +22     
==========================================
+ Hits         5931     5947      +16     
- Misses       3984     3990       +6     

Files	Coverage Δ
Sources/ConnectionPoolModule/ConnectionPool.swift	94.42% <ø> (ø)
.../Connection/PostgresConnection+Configuration.swift	50.00% <100.00%> (+0.70%)	⬆️
...urces/PostgresNIO/New/PostgresChannelHandler.swift	84.77% <100.00%> (ø)
...stgresNIO/New/PostgresFrontendMessageEncoder.swift	100.00% <100.00%> (ø)
...nection State Machine/ConnectionStateMachine.swift	62.87% <86.66%> (+0.41%)	⬆️

... and 2 files with indirect coverage changes