[beyond-did-web] neglect of pre-rotation for did:webs

[nkongsuwan]

In Section Key Rotation, the following is stated:
... the DID document needs to be updated to publish a new public key. In order to fulfill the requirements listed above, the old private key must still be accessible ...

This is not accurate for did:webs that utilizes the key pre-rotation scheme in KERI. In did:webs, two sets of keys are specified in each key event, namely the current keys and the next keys. The current keys are keys that appear in the DID Document. However, the next keys are pre-committed using hash digests in the key event. The next keys are unexposed until they are used for rotation.

Hence, the (old) current keys may be missing, and the controller can still use the next keys to perform rotation.