chore(cache): stop relying on globalThis.gc being untyped

[tomas-zijdemans]

Deno canary types globalThis.gc, so the @ts-expect-error above it is now unused and deno test fails with TS2578. Reading gc through Reflect.get works on both stable and canary, and keeps the test honest about gc being optional.

Partly solves #7219. The other 24 canary errors are a Deno-side URLPattern type collision, filed separately: denoland/deno#36346

Tested on 2.9.3 and canary 2.9.4+41e97d3, including deno test --v8-flags="--expose-gc" cache/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.00%. Comparing base (ad7c87b) to head (510c2d3).

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #7253    +/-   ##
========================================
  Coverage   95.00%   95.00%            
========================================
  Files         617      617            
  Lines       51674    51422   -252     
  Branches     9326     9277    -49     
========================================
- Hits        49093    48855   -238     
+ Misses       2038     2029     -9     
+ Partials      543      538     -5     

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