Remove redundant nested operator in builtin grok expression. (#47870)

This prevents the following warning from being printed to console:
`regular expression has redundant nested repeat operator + /%\{(?<name>(?<pattern>[A-z0-9]+)(?::(?<subname>[[:alnum:]@\[\]_:.-]+))?)(?:=(?<definition>(?:(?:[^{}]+|\.+)+)+))?\}/`

The current grok expression is not failing, but just this warning is being printed.
The warning started being printed after upgrading joni (#47374).

Closes #47861

libs/grok/src/main/java/org/elasticsearch/grok/Grok.java

@@ -54,9 +54,7 @@ public final class Grok {
             "(?::(?<subname>[[:alnum:]@\\[\\]_:.-]+))?" +
             ")" +
             "(?:=(?<definition>" +
-            "(?:" +
             "(?:[^{}]+|\\.+)+" +
-            ")+" +
             ")" +
             ")?" + "\\}";
     private static final Regex GROK_PATTERN_REGEX = new Regex(GROK_PATTERN.getBytes(StandardCharsets.UTF_8), 0,