docs/generate: remove uses of pkg/errors

[thaJeztah]

• relates to remove uses of pkg/errors in tests  #5781
• will conflict with docs: include required tools in source tree #4228

While there may be reasons to keep pkg/errors in production code, we don't need them for these tests.

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.28%. Comparing base (ecfdf74) to head (2f79598).
Report is 6 commits behind head on master.

Additional details and impacted files

@@ Coverage Diff @@## master #5784 +/- ##
=======================================
Coverage 59.28% 59.28% =======================================
Files 358 358 Lines 29830 29830 =======================================
Hits 17685 17685 Misses 11173 11173 Partials 972 972 

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