Revert Cofunction.riesz_representation argument change

[jrmaddison]

Revert solver_options to solver_parameters, consistent with release and naming elsewhere in Firedrake.

Opening as PR rather than an issue as it's a small change in previously untested code.

[connorjward]

Seems sensible.

[dham]

Breaks Gusto and Spyro so will need corresponding pull requests in those projects.

[jrmaddison]

I'd rather not make changes outside of Firedrake, would a backwards compatible FutureWarning be OK?

[dham]

Hang fire briefly. We think both of these might actually be spurious in different ways so this fix might not be needed.