Update identity 0.6 to Stronghold 2.0

[PhilippGackstatter]

Description of change

Upgrade stronghold to 2.0.

• Handle breaking changes in stronghold in a non-breaking manner. The API is not broken, however the behavior is which requires a manual migration of the old strongholds to the new snapshot format introduced in Stronghold 2.0.
• Update Cargo.lock in stronghold-nodejs which is necessary for a successful build of those bindings.
• Remove the outdated null ptr lint on support/v0.6, analogously to what was done in Remove outdated lint #1111.

Links to any relevant issues

Be sure to reference any related issues by adding fixes issue #.

Type of change

Add an x to the boxes that are relevant to your changes.

• Bug fix (a non-breaking change which fixes an issue)
• Enhancement (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Fix

How the change has been tested

This PR was tested by running the account_create on the current support/v0.6. The resulting stronghold snapshot was then used as input for migrating the snapshot from v2 to v3 using the migrate_stronghold_snapshot_v2_to_v3 example (and using "identity.rs" as the salt) from iota-sdk (currently on iotaledger/iota-sdk#456).

Then the account_create_vc example was modified to load the existing DID and the v3 snapshot using the code from this PR and signing with the existing method whose key is in the stronghold. The example ran successfully.

It was also tested that an upate to an identity can be performed, signed by a key in the migrated stronghold, published and successfully resolved.

Change checklist

Add an x to the boxes that are relevant to your changes.

• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[eike-hass]

LGTM! Merged blocked until we have a Stronghold 2.0 final though.