[flagd] migrate provider from evaluation.v1 to evaluation.v2 RPCs

[toddbaert]

The Ruby flagd provider still calls the deprecated flagd.evaluation.v1 service (client.rb requires flagd/evaluation/v1/...) while every other provider is on flagd.evaluation.v2 — migrate it to v2 (unblocks the v1 removal in open-feature/flagd#1088).

Companion: #210 (adopt the shared flagd-testbed e2e suite).

[toddbaert]

Note for whoever picks this up: while starting the shared testbed e2e suite (#210), the @no-default-variant scenarios had to be excluded because they depend on v2 semantics — for a flag with no defaultVariant, evaluation.v2 omits the (optional) value and returns reason DEFAULT, whereas evaluation.v1 returns FLAG_NOT_FOUND/ERROR. The current provider therefore reports ERROR where the testbed expects DEFAULT.

Once this migration lands, drop the and not @no-default-variant filter in the flagd provider's cucumber.yml so those scenarios run (and they should pass on v2).