Hardcode session encryption cost for migration (#2395)

**Why**: In #2353 we changed the scrypt cost with changed the scrypt
cost which affected the session encryptor causing sessions encrypted by
old and new hosts to be incompatible. This commit hardcodes the cost in
the deprecated encryptor so that the sessions will be compatible between
hosts.

app/services/encryption/encryptors/deprecated_session_encryptor.rb

@@ -18,7 +18,7 @@ def self.load_or_init_user_access_key
 
         key = Figaro.env.session_encryption_key
         user_access_key = UserAccessKey.new(
-          password: key, salt: OpenSSL::Digest::SHA256.hexdigest(key)
+          password: key, salt: OpenSSL::Digest::SHA256.hexdigest(key), cost: '4000$8$4$'
         )
         @user_access_key_scrypt_hash = user_access_key.as_scrypt_hash
         user_access_key