Drop outdated/abandoned dependencies

[remycx]

Description

https://github.com/pmezard/go-difflib : as quoted, "THIS PACKAGE IS NO LONGER MAINTAINED." ; code hasn't evolved in 9 years.
https://github.com/davecgh/go-spew : hasn't been touched in 6 years.

Removing old & dangerous code would lead to a cleaner codebase.

Proposed solution

Use case

Reduce the dependency on outdated & abandoned repositories, to improve the safety of the library, and the potential supply chain attacks.

[dolmen]

Easy to say. But what concrete solution do you suggest? Did you have a look at existing issues?

[dolmen]

@remycx Note also that I have proposed #1579 to unlink Testify from gopkg.in/yaml.v3 (which is as unmaintained as the 2 other dependencies mentioned here, and is, in my opinion, a much higher security risk, not for Testify but for the Go ecosystem in general). Your opinion and review of that PR would be welcome.