With the output of `output.txt`, open the pcap file in wireshark.

Go to Edit > Preferences > Protocols > IEEE 802.11 > Decryption Keys [Edit...]

Add key of type `wpa-pwd`: `mickeymouse:Gone_Surfing`

The traffic should now be decrypted.

... Turns out the password itself is the flag

picoCTF{mickeymouse}