fix: Respect Federation version in @link directive

[lyricsboy]

Problem

The federation version, though specifiable in the schema configuration, was not reflected in the @link directive URL when output via the federation_sdl method.

Solution

Use the federation_version as specified when building the URL.

[dzmitry-miadzvedzeu]

Hello Everyone.

I want to clarify the status of this PR, if this change is considered a "breaking change", and hence, not possible to merge right now (reference to #267)?

Thank you in advance for your time and assistance.

[Kaytal]

I know it's been a while since this PR was originally opened. However, I second this PR. It shouldn't be considered a breaking change any more than defining your federation version: 2.1 and getting a @link spec that matches federation 2.3. Additionally, locking the specification to 2.3 prevents many additional improvements and directives on later versions of Apollo Federation.