dev: use encoding/xml instead of go-xmlfmt/xmlfmt

[alexandear]

The PR refactors the Checkstyle.Print implementation to use xml.MarshalIndent instead of go-xmlfmt/xmlfmt.FormatXML. This allows dropping the github.com/go-xmlfmt/xmlfmt dependency.

[ldez]

There is no problem with github.com/go-xmlfmt/xmlfmt, so we will not replace it.

[alexandear]

There is no problem with github.com/go-xmlfmt/xmlfmt, so we will not replace it.

I proposed using xml.MarshalIndent from the Go standard library to minimize external dependencies and simplify maintenance. There is no need for an external dependency when we can utilize the standard library.

[ldez]

We don't need to minimize external dependencies.
This doesn't simplify the maintenance because this lib is stable.

And the reasons behind the usage of this lib are still relevant.