MDL-83412 core: Use LLL/LLLL for standalone Month names

We currently use MMM and MMMM which mean 'month in year', however in
some locales this is context-specific and should only be used in
conjunction with a day, for example "10 October 2024".

LLL and LLLL are for "stand-alone on the in year" and should be used
when a month is used without a day.

lib/classes/date.php

@@ -741,13 +741,14 @@ class core_date {
         $intl_formats = [
           '%a' => 'EEE',	// An abbreviated textual representation of the day	Sun through Sat
           '%A' => 'EEEE',	// A full textual representation of the day	Sunday through Saturday
-          '%b' => 'MMM',	// Abbreviated month name, based on the locale	Jan through Dec
-          '%B' => 'MMMM',	// Full month name, based on the locale	January through December
-          '%h' => 'MMM',	// Abbreviated month name, based on the locale (an alias of %b)	Jan through Dec
         ];
 
-        $intl_formatter = function (DateTimeInterface $timestamp, string $format) use ($intl_formats, $locale) {
-
+        $originalformat = $format;
+        $intl_formatter = function (DateTimeInterface $timestamp, string $format) use (
+            $intl_formats,
+            $locale,
+            $originalformat,
+        ) {
           // Map IANA timezone DB names (used by PHP) to those used internally by the "intl" extension. The extension uses its
           // own data based on ICU timezones, which may not necessarily be in-sync with IANA depending on the version installed
           // on the local system. See: https://unicode-org.github.io/icu/userguide/datetime/timezone/#updating-the-time-zone-data
@@ -789,6 +790,19 @@ class core_date {
               $time_type = IntlDateFormatter::MEDIUM;
               break;
 
+            case "%B":
+            case "%b":
+            case "%h":
+                // Check for any day (%d, or %e) in the string.
+                if (preg_match('/%[de]/', $originalformat)) {
+                    // The day is present so use the standard format.
+                    $pattern = $format === '%B' ? 'MMMM' : 'MMM';
+                } else {
+                    // The day is not present so use the stand-alone format.
+                    $pattern = $format === '%B' ? 'LLLL' : 'LLL';
+                }
+                break;
+
             default:
               $pattern = $intl_formats[$format];
           }