Run go fix

[mcpherrinm]

This runs the updated go fix ./… from Go 1.26

There are some more fixes once Boulder updates to Go 1.26 - these are just for
the current Go 1.25.

Fixes applied:

• Use strings.Cut instead of strings.Index and manual slicing
• Remove some obsolete tc := tc captures
• Use a strings.Builder to avoid quadratic string building
• Use reflect.TypeFor instead of reflect.TypeOf

[aarongable]

I was planning on waiting until go1.26 to run this tool, but LGTM.

Could you provide more details about exactly what go fix command(s) you ran? Did you limit it to certain analyzers or certain files?

[mcpherrinm]

The invocation was go fix ./… - all fixes on all the files. Updated PR description to make that more explicit