chore: bump golangci-lint version, ignore _gen.go files

1.19 has issues on the GolangCI service[1], which is fixed by upgrading.

[1]: https://golangci.com/r/github.com/loadimpact/k6/pulls/1143

.golangci.yml

@@ -17,7 +17,7 @@ issues:
   exclude-rules:
    # Exclude duplicate code and function length and complexity checking in test
    # files (due to common repeats and long functions in test code)
-   - path: _test\.go #TODO: also exclude all go-generated files by giving them common names? *_gen.go?
+   - path: _(test|gen)\.go
      linters:
        - gocyclo
        - dupl
@@ -57,4 +57,4 @@ linters:
   fast: false
 
 service:
-  golangci-lint-version: 1.20.x  #TODO: upgrade to 1.20
+  golangci-lint-version: 1.20.x