Replace jsrsasign dependency

Dependency "jsrsasign" will shortly go out of support:

https://github.com/kjur/jsrsasign/blob/master/README.md#end-of-support-announcement-for-jsrsasign

> End of Support Announcement for jsrsasign
On 14 April 2026, we announce end of support for jsrsasign.
>
> Effective 3 Jun 2026, support is no longer provided for jsrsasign and all version of "npm" packages of jsrsasign is deprecated. Thank you very much for using jsrsasign since the release of its first version, 1.0, on June 3, 2010.

A new package should be found to replace this dependency.

Start by locating jsrsasign in the package manifest and tracing every use of it across CyberChef. Review the linked end-of-support announcement, compare viable replacement packages against the current operations, and identify the relevant existing checks before deciding on the replacement. Done means jsrsasign is removed and the affected functionality remains covered and working.

Written by the indexing model from the issue text.