fix: precision of InstantPatternLegacyFormatter

To get the correct precision in the `InstantPatternLegacyFormatter` we need to replace the legacy `n` pattern letter with its `DateTimeFormatter` equivalent `S`.

Author: ppkarwasz

log4j-core-test/src/test/java/org/apache/logging/log4j/core/pattern/NamedInstantPatternTest.java

@@ -41,5 +41,6 @@ void compatibilityOfLegacyPattern(NamedInstantPattern namedPattern) {
         MutableInstant instant = new MutableInstant();
         instant.initFromEpochSecond(javaTimeInstant.getEpochSecond(), javaTimeInstant.getNano());
         assertThat(legacyFormatter.format(instant)).isEqualTo(formatter.format(instant));
+        assertThat(legacyFormatter.getPrecision()).isEqualTo(formatter.getPrecision());
     }
 }

log4j-core/src/main/java/org/apache/logging/log4j/core/util/internal/instant/InstantPatternLegacyFormatter.java

@@ -46,7 +46,10 @@ final class InstantPatternLegacyFormatter implements InstantPatternFormatter {
     private final BiConsumer<StringBuilder, Instant> formatter;
 
     InstantPatternLegacyFormatter(final String pattern, final Locale locale, final TimeZone timeZone) {
-        this.precision = new InstantPatternDynamicFormatter(pattern, locale, timeZone).getPrecision();
+        // Replaces 'n' used in legacy patterns with 'S' to obtain the right precision.
+        // In legacy patterns, the precision of 'n' depends on the pattern length, but
+        // in `DateTimeFormatter`, it is always nanoseconds.
+        this.precision = new InstantPatternDynamicFormatter(pattern.replace('n', 'S'), locale, timeZone).getPrecision();
         this.pattern = pattern;
         this.locale = locale;
         this.timeZone = timeZone;