WolfSSL reference outdated

[jan-kiszka]

No one updated the submodule reference to WolfSSL since 1b35000. That version 4.6.0 has some open CVEs, I didn't check if they would apply, but I did check if 5.6.3 works as well - and it seems so. Anyone willing to push that? I would, but there is a "Contribution Limitation Agreement" in place here, and that is a bit in imbalance to such simple changes (although I may have a second one, see https://groups.google.com/d/msgid/isar-users/424e2519-2dfa-5379-a16e-ca505e6135fe%40siemens.com).