fix (devices): use ugm3 abbreviation consistently

These ones used the Greek mu for micro, but at least one was using
3 without superscript so was wrong. Avoid issues by using the ASCII
abbreviation that is converted in code using the HA constant.

custom_components/tuya_local/devices/pv28-cw_airquality_monitor.yaml

@@ -239,7 +239,7 @@ entities:
       - id: 105
         type: integer
         name: value
-        unit: "μg/m3"
+        unit: ugm3
         range:
           min: 15
           max: 75

custom_components/tuya_local/devices/vestfrost_vp_a1z40hw_purifier.yaml

@@ -33,7 +33,7 @@ entities:
         class: measurement
         name: sensor
         type: integer
-        unit: μg/m³
+        unit: ugm3
   - entity: sensor
     name: Filter remaining
     category: diagnostic
@@ -99,7 +99,7 @@ entities:
         class: measurement
         name: sensor
         type: integer
-        unit: μg/m³
+        unit: ugm3
   - entity: sensor
     class: pm1
     dps:
@@ -107,7 +107,7 @@ entities:
         class: measurement
         name: sensor
         type: integer
-        unit: μg/m³
+        unit: ugm3
   - entity: select
     category: config
     name: Display