Handle excess objects correctly in FindObject

[jsha]

Currently, we are relying on a "more" boolean returned from
FindObjects. But according to
https://pkg.go.dev/github.com/miekg/pkcs11?tab=doc#Ctx.FindObjects,

The returned boolean value is deprecated and should be ignored.

Instead, we should ask for more objects than we need and error if we get
more than 1.

Related: letsencrypt/boulder#4972