feat(exporter-zipkin)!: do not read environment variables from window

[pichlermarc]

Which problem is this PR solving?

Migrates from getEnv() to using the new get*FromEnv() methods added in #5443. Also inlines the defaults. This drops
"env var configuration" (via window.OTEL_*) in browser environments.

This breaking change now will enable us to split the env var config code completely in a non-breaking feature release later.

Refs #5217

Type of change

• Breaking change (fix or feature that would cause existing functionality to not work as expected)

How Has This Been Tested?

• Existing tests (browser behavior for this was untested, so nothing is added or removed)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.94%. Comparing base (5c90aec) to head (5097658).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5465   +/-   ##
=======================================
  Coverage   94.94%   94.94%           
=======================================
  Files         309      309           
  Lines        8013     8016    +3     
  Branches     1697     1699    +2     
=======================================
+ Hits         7608     7611    +3     
  Misses        405      405           

Files with missing lines	Coverage Δ
...ckages/opentelemetry-exporter-zipkin/src/zipkin.ts	100.00% <100.00%> (ø)