Bump @ethersproject/signing-key dependency from 5.7.0 to 6.x

[pirj]

Dependabot complains that:

@openzeppelin/[email protected] requires [email protected] via a transitive dependency on @ethersproject/[email protected]

If I'm reading it right, the CVE severity is set as critical.

This is fixed in elliptic 6.5.7.

I'm uncertain if the ethersproject has combined all their packages into one, or dropped their dependency on elliptic, but it doesn't appear in their 6.x package declaration anymore.

The latest @ethersproject/signing-key in their 5.7.x series, 5.7.2, still depends on the same elliptic version.

The latest [email protected] (and the master branch) depend on ethersproject dependencies 5.7.

How much of a stretch would be to bump ethersproject's dependencies to 6.x?

[frangio]

Thanks @pirj. We're removing this dependency in #56.

Please note that the elliptic package is not used by @openzeppelin/merkle-tree even though it's included among the transitive dependencies, so it's not affected by the vulnerability.