Fix ReplacementAccentSystem out of range exception (#16786)

Content.Server/Speech/EntitySystems/ReplacementAccentSystem.cs

@@ -53,8 +53,10 @@ namespace Content.Server.Speech.EntitySystems
                 // this is kind of slow but its not that bad
                 // essentially: go over all matches, try to match capitalization where possible, then replace
                 // rather than using regex.replace
-                foreach (Match match in Regex.Matches(message, $@"(?<!\w){f}(?!\w)", RegexOptions.IgnoreCase))
+                for (int i = Regex.Count(message, $@"(?<!\w){f}(?!\w)", RegexOptions.IgnoreCase); i > 0; i--)
                 {
+                    // fetch the match again as the character indices may have changed
+                    Match match = Regex.Match(message, $@"(?<!\w){f}(?!\w)", RegexOptions.IgnoreCase);
                     var replacement = r;
 
                     // Intelligently replace capitalization