Replace custom `X5cKeyFinder` with `JWT::X5cKeyFinder`

[nicolastemciuc]

We currently maintain our own X5cKeyFinder implementation. However, the jwt gem has added a built-in JWT::X5cKeyFinder, which provides the same functionality as part of the core library.

To reduce duplication and maintenance overhead, we should remove our custom implementation and switch to using the official JWT::X5cKeyFinder.