Remove credit-only account handling (#6726)

* Renaming
- credit-only/credit-debit to read-only/read-write
- debitable to writable

* Remove credit handling, making credit-only accounts read-only

* Update programs to remove deprecated credit-only account designation

* Use readonly and writable instead of underscored types

programs/librapay_api/src/librapay_instruction.rs

@@ -36,7 +36,7 @@ pub fn mint(
     let ix_data = LoaderInstruction::InvokeMain { data };
 
     let accounts = vec![
-        AccountMeta::new_credit_only(*program_pubkey, false),
+        AccountMeta::new_readonly(*program_pubkey, false),
         AccountMeta::new(*from_pubkey, true),
         AccountMeta::new(*to_pubkey, false),
     ];
@@ -65,8 +65,8 @@ pub fn transfer(
     let ix_data = LoaderInstruction::InvokeMain { data };
 
     let accounts = vec![
-        AccountMeta::new_credit_only(*program_pubkey, false),
-        AccountMeta::new_credit_only(*mint_pubkey, false),
+        AccountMeta::new_readonly(*program_pubkey, false),
+        AccountMeta::new_readonly(*mint_pubkey, false),
         AccountMeta::new(*from_pubkey, true),
         AccountMeta::new(*to_pubkey, false),
     ];