RUSTSEC-2020-0095: difference is unmaintained

[github-actions]

difference is unmaintained

Details
Status	unmaintained
Package	difference
Version	2.0.0
URL	johannhof/difference.rs#45
Date	2020-12-20

The author of the difference crate is unresponsive.

Maintained alternatives:

• dissimilar

• treediff

• diffus

See advisory page for additional details.

[ClementTsang]

Can't really fix this, and checking cargo tree -p difference -i, we see this is only used in dev-dependencies for testing:

➜ cargo tree -p difference -i
difference v2.0.0
└── predicates v1.0.6
    └── assert_cmd v1.0.2
        [dev-dependencies]
        └── bottom v0.5.6 (/home/pyreko/projects/bottom)
    [dev-dependencies]
    └── bottom v0.5.6 (/home/pyreko/projects/bottom)

As such, marking this as wontfix and closing for now.