rpc: return warnings as an array instead of just a single one

The RPC documentation for `getblockchaininfo`, `getmininginfo` and
`getnetworkinfo` states that "warnings" returns "any network and
blockchain warnings". In practice, only a single warning is returned.

Fix that by returning all warnings as an array.

As a side benefit, cleans up the GetWarnings() logic.

src/node/interfaces.cpp

@@ -47,6 +47,7 @@
 #include <util/check.h>
 #include <util/result.h>
 #include <util/signalinterrupt.h>
+#include <util/string.h>
 #include <util/translation.h>
 #include <validation.h>
 #include <validationinterface.h>
@@ -91,7 +92,7 @@ public:
     explicit NodeImpl(NodeContext& context) { setContext(&context); }
     void initLogging() override { InitLogging(args()); }
     void initParameterInteraction() override { InitParameterInteraction(args()); }
-    bilingual_str getWarnings() override { return GetWarnings(true); }
+    bilingual_str getWarnings() override { return Join(GetWarnings(), Untranslated("<hr />")); }
     int getExitStatus() override { return Assert(m_context)->exit_status.load(); }
     uint32_t getLogCategories() override { return LogInstance().GetCategoryMask(); }
     bool baseInitialize() override