Update the command 'version'. (#8312)

# Description

No real changes, just some cleanup while I was looking at the code of
the command.

# User-Facing Changes

Remove the attribute 'pkg_version', since it's already exposed through
'version'.

# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

tests/shell/pipeline/commands/external.rs

@@ -362,7 +362,6 @@ mod nu_commands {
         assert!(actual.out.contains("version"));
         assert!(actual.out.contains("rust_version"));
         assert!(actual.out.contains("rust_channel"));
-        assert!(actual.out.contains("pkg_version"));
     }
 
     #[test]
@@ -374,7 +373,6 @@ mod nu_commands {
         assert!(actual.out.contains("version"));
         assert!(actual.out.contains("rust_version"));
         assert!(actual.out.contains("rust_channel"));
-        assert!(actual.out.contains("pkg_version"));
     }
 }