feat(core): drop getEnv(), getEnvWithoutDefaults()

[pichlermarc]

Which problem is this PR solving?

Drops getEnv(), getEnvWithoutDefaults() and associated types in favor of the new get*FromEnv() functions introduced in #5443

Fixes #5217

Type of change

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

How Has This Been Tested?

• Unit tests

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.96%. Comparing base (e9d3c71) to head (ed592d2).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5481      +/-   ##
==========================================
- Coverage   94.97%   94.96%   -0.01%     
==========================================
  Files         309      309              
  Lines        8017     7952      -65     
  Branches     1706     1687      -19     
==========================================
- Hits         7614     7552      -62     
+ Misses        403      400       -3     

Files with missing lines	Coverage Δ
...pentelemetry-core/src/platform/node/environment.ts	100.00% <ø> (+2.70%)	⬆️
...ckages/opentelemetry-core/src/utils/environment.ts	100.00% <ø> (+3.17%)	⬆️