Update .gitignore generated by 'mix new' (#13871)

The .gitignore file would reference '/.fetch' files, which caused some
confusion (cf.
https://elixir-lang.slack.com/archives/C03EPRA3B/p1727706411018609).

Some research suggests that these files are no longer generated as of
commit 59b6e2e so let's stop mentioning
them in the .gitignore, too.

lib/mix/lib/mix/tasks/new.ex

@@ -311,9 +311,6 @@ defmodule Mix.Tasks.New do
   # Where third-party dependencies like ExDoc output generated docs.
   /doc/
 
-  # Ignore .fetch files in case you like to edit your project deps locally.
-  /.fetch
-
   # If the VM crashes, it generates a dump, let's ignore it too.
   erl_crash.dump