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/shared/calendarspec.c

@@ -649,7 +649,7 @@ static int prepend_component(const char **p, bool usec, unsigned nesting, Calend
                 if (repeat == 0)
                         return -ERANGE;
         } else {
-                /* If no repeat value is specified for the µs component, then let's explicitly refuse ranges
+                /* If no repeat value is specified for the μs component, then let's explicitly refuse ranges
                  * below 1s because our default repeat granularity is beyond that. */
 
                 /* Overflow check */

src/shared/efi-loader.c

@@ -309,7 +309,7 @@ int efi_loader_get_config_timeout_one_shot(usec_t *ret) {
                 return -ERANGE;
 
         cache_stat = new_stat;
-        *ret = cache = sec * USEC_PER_SEC; /* return in µs */
+        *ret = cache = sec * USEC_PER_SEC; /* return in μs */
         return 0;
 }