替换ThreadLocal<SimpleDateFormat>以适配Java21虚拟线程

[shichaoyuan]

Issue Description

Sentinel/sentinel-core/src/main/java/com/alibaba/csp/sentinel/eagleeye/EagleEyeCoreUtils.java

Lines 142 to 148 in a524ab3

	private static final ThreadLocal<FastDateFormat> dateFmt = new ThreadLocal<FastDateFormat>() {
	@Override
	protected FastDateFormat initialValue() {
	return new FastDateFormat();
	}
	};

Sentinel/sentinel-core/src/main/java/com/alibaba/csp/sentinel/log/jul/CspFormatter.java

Lines 28 to 34 in a524ab3

	private final ThreadLocal<SimpleDateFormat> dateFormatThreadLocal = new ThreadLocal<SimpleDateFormat>() {
	@Override
	public SimpleDateFormat initialValue() {
	return new SimpleDateFormat("yyyy-MM-dd HH:mm:ss.SSS");
	}
	};

Sentinel/sentinel-core/src/main/java/com/alibaba/csp/sentinel/log/jul/DateFileLogHandler.java

Lines 36 to 42 in a524ab3

	private final ThreadLocal<SimpleDateFormat> dateFormatThreadLocal = new ThreadLocal<SimpleDateFormat>() {
	@Override
	public SimpleDateFormat initialValue() {
	return new SimpleDateFormat("yyyy-MM-dd");
	}
	};

代码中的ThreadLocal对Java21中的 virtual thread 并不友好。

而且从Java8开始已经推荐使用 java.time.format.DateTimeFormatter，这是一个 "This class is immutable and thread-safe." 的实现。

Describe your initial design

是否可以将这些地方换成 java.time.format.DateTimeFormatter？

如果可以的话，我愿意提PR

[robberphex]

是可以这样修改的，欢迎提交这个PR。