Replaced usage of deprecated Long class constructor to fix warnings.

[23rd]

Hi, I had tried build with OpenJDK 17 — seems to be good but I got two warnings about the deprecated constructor. So this small PR fixes them.
https://docs.oracle.com/javase/9/docs/api/java/lang/Long.html#Long-long-

Logs:

chatty\src\chatty\gui\components\settings\SliderLongSetting.java:66: warning: [removal] Long(long) in Long has been deprecated and marked for removal
        valueLabel.setText(LABEL_PREFIX+new Long(value).toString());
                                        ^
chatty\src\chatty\util\SimpleCache.java:53: warning: [removal] Long(long) in Long has been deprecated and marked for removal
            writer.write(new Long(System.currentTimeMillis() / 1000).toString()+"\n");
                         ^

[tedyoung]

Why not just use String concatenation? No need for the Long wrapper at all, e.g.:

valueLabel.setText(LABEL_PREFIX + value);