tree-wide: when in doubt use greek small letter mu rather than micro symbol

Doesn't really matter since the two unicode symbols are supposedly
equivalent, but let's better follow the unicode recommendations to
prefer greek small letter mu, as per:

https://www.unicode.org/reports/tr25

src/basic/time-util.c

@@ -327,7 +327,7 @@ char *format_timestamp_style(
                 if (l < (size_t) (1 + 1 + 1))
                         return NULL; /* not enough space for even the shortest of forms */
 
-                return snprintf_ok(buf, l, "@" USEC_FMT, t / USEC_PER_SEC);  /* round down µs → s */
+                return snprintf_ok(buf, l, "@" USEC_FMT, t / USEC_PER_SEC);  /* round down μs → s */
         }
 
         utc = IN_SET(style, TIMESTAMP_UTC, TIMESTAMP_US_UTC, TIMESTAMP_DATE);