Description of change

Update dependency iota-stronghold to version 2.1.0.

Change checklist

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