use an easier-to-read date format in prompt (#9585)

# Description

Switches the right prompt to use an easier-to-read date format
(YYYY-MM-DD)

It's currently using (MM-DD-YYYY) which is only used in a few countries
but it is unfortunately ambiguous with countries that use (DD-MM-YYYY)
format (most countries). While YYYY-MM-DD is used in fewer countries
than DD-MM-YYYY it's at least unambiguous to MM-DD-YYYY countries.

More info: https://en.wikipedia.org/wiki/Date_format_by_country

Alternatively, we could spell out the month name (even abbreviated) to
make it clear as well.

# User-Facing Changes

Just changes the date in the default right prompt

# Tests + Formatting
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crates/nu-utils/src/sample_config/default_env.nu

@@ -29,7 +29,7 @@ def create_right_prompt [] {
     let time_segment = ([
         (ansi reset)
         (ansi magenta)
-        (date now | date format '%m/%d/%Y %r')
+        (date now | date format '%Y/%m/%d %r')
     ] | str join | str replace --all "([/:])" $"(ansi green)${1}(ansi magenta)" |
         str replace --all "([AP]M)" $"(ansi magenta_underline)${1}")