Update bootloaders.json - tinyUF2 0.21.0

To pick up https://github.com/adafruit/tinyuf2/releases/tag/0.21.0

Author: tyeth

_data/bootloaders.json

@@ -7,10 +7,10 @@
             "version": "v3.16.0"
         },
         "esp32s2": {
-            "version": "0.20.1"
+            "version": "0.21.0"
         },
         "esp32s3": {
-            "version": "0.20.1"
+            "version": "0.21.0"
         },
         "esp32c2": {},
         "esp32c3": {},