rpc: use importdescriptors with Core >= 0.21

Only use the old `importmulti` with Core versions that don't support
descriptor-based (sqlite) wallets.

Add an extra feature to test against Core 0.20 called `test-rpc-legacy`

.github/workflows/cont_integration.yml

@@ -95,6 +95,9 @@ jobs:
           - name: rpc
             testprefix: blockchain::rpc::test
             features: test-rpc
+          - name: rpc-legacy
+            testprefix: blockchain::rpc::test
+            features: test-rpc-legacy
           - name: esplora
             testprefix: esplora
             features: test-esplora,use-esplora-reqwest,verify