clang-tidy: Apply modernize-deprecated-headers

This can be reproduced according to the developer notes with something
like

( cd ./src/ && ../contrib/devtools/run-clang-tidy.py -p ../bld-cmake -fix -j $(nproc) )

Also, the header related changes were done manually.

src/key_io.cpp

@@ -12,8 +12,8 @@
 #include <util/strencodings.h>
 
 #include <algorithm>
-#include <assert.h>
-#include <string.h>
+#include <cassert>
+#include <cstring>
 
 /// Maximum witness length for Bech32 addresses.
 static constexpr std::size_t BECH32_WITNESS_PROG_MAX_LEN = 40;