Use supports_add_constrained_variables

[blegat]

supports_add_constrained_variables was introduced in a previous MOI release and it is now required to implement it rather than supports_constraint as the LazyBridgeOptimizer now assumes that this is implemented by solvers.
This is a breaking change for solvers (not for users). We shouldn't really have included it in a non-breaking MOI release but we've chosen the approach of introducing the new function in an early release to leave the time for solvers to transition. Now is the time as MOI v0.9.15 will start requiring the solvers to have tranitioned.
This PR does the transition.

[ericphanson]

Thanks very much for the PR!

Looking at the CI logs,

• the nightly failures seem to be a Julia issue
• It seems like somehow the presolve isn't working as well now? (not sure how that could be related though)
• some of the @test_throws errors could be updated from BoundsError to an MOI unsupported error
• It looks like though there are also MOI unsupported errors for things that didn't throw before, e.g. https://github.com/ericphanson/SDPAFamily.jl/pull/38/checks?check_run_id=1068890156#step:5:262

[blegat]

It looks like though there are also MOI unsupported errors for things that didn't throw before

I just updated the commit, this should fix this.

[ericphanson]

looks good, thanks!