update headers for BIP70 payment request

Summary:
Badger Cash and RaiPay use "ecash-" instead of "bitcoincash-" in the headers.
I'm also going to apply the same change to Electrum ABC Bitcoin-ABC/ElectrumABC#213

Test Plan:
- Generate a payment URI: `curl -L -H 'Content-Type: application/json' -d '{"outputs":[{"address":"1NLcNpAaBBMekgBZk7NxwdxwtSUTfTV8Aq","amount":560}],"currency":"ecash"}' https://pay.badger.cash/create_invoice`
- Run `src/qt/bitcoin-qt`
- Menu `File` > `Open URI`
- Paste the "ecash:?r=<url>" (replace <url> with the url in the reply to the curl command, e.g. https://pay.badger.cash/i/DTKP4)
- Send

Repeat with the be.cash API:
`curl -L -H 'Content-Type: application/json' -d '{"values":{"network":"XEC","outputs":[{"value":3000,"scriptHex":"76a914154205358162a6f59457d83dea828a15261aaba388ac"}]}}' https://p.be.cash/create-invoice`

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11543

doc/release-notes.md

@@ -5,3 +5,10 @@ Bitcoin ABC version 0.25.7 is now available from:
   <https://download.bitcoinabc.org/0.25.7/>
 
 This release includes the following features and fixes:
+- Redefine the BIP71 MIME types for eCash payment request messages to:
+
+    | Message        | Type/Subtype                     |
+    |----------------|----------------------------------|
+    | PaymentRequest | application/ecash-paymentrequest |
+    | Payment        | application/ecash-payment        |
+    | PaymentACK     | application/ecash-paymentack     |

src/qt/paymentserver.cpp

@@ -50,10 +50,9 @@ const int BITCOIN_IPC_CONNECT_TIMEOUT = 1000; // milliseconds
 const char *BIP70_MESSAGE_PAYMENTACK = "PaymentACK";
 const char *BIP70_MESSAGE_PAYMENTREQUEST = "PaymentRequest";
 // BIP71 payment protocol media types
-const char *BIP71_MIMETYPE_PAYMENT = "application/bitcoincash-payment";
-const char *BIP71_MIMETYPE_PAYMENTACK = "application/bitcoincash-paymentack";
-const char *BIP71_MIMETYPE_PAYMENTREQUEST =
-    "application/bitcoincash-paymentrequest";
+const char *BIP71_MIMETYPE_PAYMENT = "application/ecash-payment";
+const char *BIP71_MIMETYPE_PAYMENTACK = "application/ecash-paymentack";
+const char *BIP71_MIMETYPE_PAYMENTREQUEST = "application/ecash-paymentrequest";
 #endif
 
 //