JP frequency - 20mW limit, change freqs to avoid duty cycle

Thanks to user Goyath on Discord, we discovered that in Japan the
250mW radio level requires licensing, and 20mW is the practical
limit. We also discovered that a duty cycle of 10% is needed on
most frequencies.

    CH 24-38 920.5-923.5 20mW no airtime restrictions
    CH 39-61 923.5-928.1 20mW 10% airtime

src/mesh/RadioInterface.cpp

@@ -53,8 +53,10 @@ const RegionInfo regions[] = {
 
     /*
         https://lora-alliance.org/wp-content/uploads/2020/11/lorawan_regional_parameters_v1.0.3reva_0.pdf
+        https://www.arib.or.jp/english/html/overview/doc/5-STD-T108v1_5-E1.pdf
+        https://qiita.com/ammo0613/items/d952154f1195b64dc29f
      */
-    RDEF(JP, 920.8f, 927.8f, 100, 0, 16, true, false, false),
+    RDEF(JP, 920.5f, 923.5f, 100, 0, 13, true, false, false),
 
     /*
         https://www.iot.org.au/wp/wp-content/uploads/2016/12/IoTSpectrumFactSheet.pdf
@@ -585,4 +587,4 @@ size_t RadioInterface::beginSending(meshtastic_MeshPacket *p)
 
     sendingPacket = p;
     return p->encrypted.size + sizeof(PacketHeader);
-}
+}