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.

doc/release-notes-29845.md

@@ -0,0 +1,8 @@
+RPC
+---
+
+- the `warnings` field in `getblockchaininfo`, `getmininginfo` and
+  `getnetworkinfo` now returns all the active node warnings as an array
+  of strings, instead of just a single warning. The current behaviour
+  can temporarily be restored by running bitcoind with configuration
+  option `-deprecatedrpc=warnings`.