fix(strings): correct Cyrillic hard/soft sign mappings to match original behavior

The Cyrillic hard and soft signs (Ъ, ъ, Ь, ь) were incorrectly mapped to empty strings in cyrillic.json.
This fix restores the correct mappings from the original Utf8ToAsciiConverter implementation:
- Ъ (hard sign uppercase) → " (double quote)
- ъ (hard sign lowercase) → " (double quote)
- Ь (soft sign uppercase) → ' (single quote)
- ь (soft sign lowercase) → ' (single quote)

These mappings now match the golden-mappings.json reference file extracted from the original implementation.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/Umbraco.Core/Strings/CharacterMappings/cyrillic.json

@@ -57,12 +57,12 @@
     "ш": "sh",
     "Щ": "Shch",
     "щ": "shch",
-    "Ъ": "",
-    "ъ": "",
+    "Ъ": "\"",
+    "ъ": "\"",
     "Ы": "Y",
     "ы": "y",
-    "Ь": "",
-    "ь": "",
+    "Ь": "'",
+    "ь": "'",
     "Э": "E",
     "э": "e",
     "Ю": "Yu",