clientconn: close canceled transport synchronously

[twz123]

When a connection attempt is canceled after the transport is created, ensure that it is closed inline. This prevents untracked goroutines from being left behind.

See:

• Allow waiting for all goroutines to exit on client connection close #8655
• clientconn: Wait for all goroutines on close #8666 (comment)

RELEASE NOTES:

• When canceling a connection attempt, closing the transport won't use an untracked goroutine anymore.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.29%. Comparing base (4046676) to head (5ccc206).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8786      +/-   ##
==========================================
- Coverage   83.42%   83.29%   -0.13%     
==========================================
  Files         418      418              
  Lines       32897    32898       +1     
==========================================
- Hits        27443    27402      -41     
- Misses       4069     4093      +24     
- Partials     1385     1403      +18     

Files with missing lines	Coverage Δ
clientconn.go	90.81% <100.00%> (+0.12%)	⬆️

... and 23 files with indirect coverage changes

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