chore(build): upgrade Node CI runtimes and remove Node 18 pnpmfile patch

[quirogas]

Update Node 14 and 18 matrix configurations across GitHub Actions workflows to
Node 20 and 22 , update generator engine constraints, and remove the transitional root .pnpmfile.cjs compatibility patch.

With Node 18 officially removed from the Node repo retaining test matrices targeting older runtime
versions creates unnecessary maintenance friction and prevents modernizing shared development
dependencies. Furthermore, the root .pnpmfile.cjs hook is no longer required. Removing this file and
stripping --pnpmfile injection flags from ci/run_single_test.sh simplifies execution scripts.

For googleapis/librarian#5095
For googleapis/librarian#6353

[gemini-code-assist]

Code Review

This pull request removes the .pnpmfile.cjs file, simplifies the dependency installation in ci/run_single_test.sh by removing the --pnpmfile flag, and updates the Node.js engine requirements in several package.json files. The reviewer notes that duplicate_methods_test-esm/package.json is an auto-generated baseline file and should not be edited manually; instead, the engine constraints should be updated upstream in the generator or templates.

[gemini-code-assist]

This file appears to be an auto-generated baseline file. Manually editing auto-generated files is discouraged as these changes will be overwritten during the next regeneration. Please apply the engine constraint updates upstream in the generator or templates, and then regenerate the baselines using the appropriate generation script.

References

1. Do not manually edit auto-generated files to fix typos or make other changes, as these edits will be overwritten during the next regeneration. Instead, apply the fixes upstream in the generator or templates.