refactor(auto-instrumentations-node): use env var utility functions to setup diagnostics logging

[danherd]

Which problem is this PR solving?

With open-telemetry/opentelemetry-js#5443 we've introduced new utility functions for getting getting environment variables in an OTel Spec compliant manner, and we've migrated from using the old getEnv() utility function (#2708).

However, to provide an easier upgrade-path, for users of getEnv() we've also introduced a utility function to get a log level from a string, so the mapping we now do is now obsolete and we should align it with the core repo.

Refs #2774

Short description of the changes

Removed unnecessary utility function, map and tests. Replaced with new core utility functions.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: danherd / name: Dan Herd (56a7636, 6f148e9, 417f37f)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.50%. Comparing base (d2c1be4) to head (6f148e9).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2778      +/-   ##
==========================================
+ Coverage   89.45%   89.50%   +0.05%     
==========================================
  Files         174      174              
  Lines        8324     8318       -6     
  Branches     1593     1591       -2     
==========================================
- Hits         7446     7445       -1     
+ Misses        878      873       -5     

Files with missing lines	Coverage Δ
...tapackages/auto-instrumentations-node/src/utils.ts	98.23% <100.00%> (-0.09%)	⬇️

... and 2 files with indirect coverage changes

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