introduce and use the generalized node::Warnings interface

Instead of having separate warning functions (and globals) for each
different warning that can be raised, encapsulate this logic into
a single class and allow to (un)set any number of warnings.

Introduces behaviour change:
- the `-alertnotify` command is executed for all
  `KernelNotifications::warningSet` calls, which now also covers the
  `LARGE_WORK_INVALID_CHAIN` warning.
- previously, warnings were returned based on a predetermined order,
  e.g. with the "pre-release test build" warning always first. This
  is no longer the case, and Warnings::GetMessages() will return
  messages sorted by the id of the warning.

Removes warnings.cpp from kernel.

src/kernel/warning.h

@@ -0,0 +1,14 @@
+// Copyright (c) 2024-present The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_KERNEL_WARNING_H
+#define BITCOIN_KERNEL_WARNING_H
+
+namespace kernel {
+enum class Warning {
+    UNKNOWN_NEW_RULES_ACTIVATED,
+    LARGE_WORK_INVALID_CHAIN,
+};
+} // namespace kernel
+#endif // BITCOIN_KERNEL_WARNING_H