refactor: fix modernize lint issues

This MR enables the modernize linter in Golangci-lint config.

And fixes the following lint issues:

$ golangci-lint run
pagination.go:2:1: buildtag: +build line is no longer needed (govet)
pagination_test.go:2:1: buildtag: +build line is no longer needed (govet)
markdown_test.go:39:3: forvar: copying variable is unneeded (modernize)
project_variables_test.go:117:42: any: interface{} can be replaced by any (modernize)
request_handler.go:118:27: reflecttypefor: reflect.TypeOf call can be simplified using TypeFor (modernize)
request_handler_test.go:221:26: any: interface{} can be replaced by any (modernize)
6 issues:
* govet: 2
* modernize: 4

The check modernize.omitzero is disabled because the fix requires backward-incompatible changes.