share/log: change log_syntax from "[a:b] " to "a🅱️ "

Those square brackets don't fit how our other messages look like; we use colons
everywhere else. The "[a:b]" format was originally added in
ed5bcfbe3c, and remained unchanged for 7 years,
but in the meantime other conventions evolved.

The new version is also one character shorter.

[/etc/systemd/system/systemd-networkd.service.d/override.conf:2] Failed to parse sec value, ignoring: ...
  ↓
/etc/systemd/system/systemd-networkd.service.d/override.conf:2: Failed to parse sec value, ignoring: ...

src/basic/log.c

@@ -1162,7 +1162,7 @@ int log_syntax_internal(
                         LOG_MESSAGE_ID(SD_MESSAGE_INVALID_CONFIGURATION),
                         "CONFIG_FILE=%s", config_file,
                         "CONFIG_LINE=%u", config_line,
-                        LOG_MESSAGE("[%s:%u] %s", config_file, config_line, buffer),
+                        LOG_MESSAGE("%s:%u: %s", config_file, config_line, buffer),
                         unit_fmt, unit,
                         NULL);
 }