feat: updates harmonizer to v0.30.0

[EverlastingBugstopper]

fixes #801 by updating harmonizer to v0.30.0, which is a breaking change for folks using rover supergraph compose with versions of @apollo/gateway < v0.39.0. This is reflected in the docs and will also be called out in our changelog.

cc @StephenBarlow for docs wording/structure review

cc @trevor-scheer for content review

cc @abernix for rubber stamp

cc @prasek because supergraph-demo fails to compose with this version bump.

[trevor-scheer]

Changes LGTM! (reviewed docs content only)

[EverlastingBugstopper]

Thanks @abernix - I like what you wrote quite a bit more than what I wrote!

[EverlastingBugstopper]

@abernix I've added a section to the docs calling out additional requirements for the @tag directive that were called out in the changelog here and satisfied in this supergraph-demo PR.

@trevor-scheer was helpful here and noted that this is just a temporary breaking change and future versions will not have these additional requirements

[trevor-scheer]

Unbreak will happen via this PR: apollographql/federation#1022

[EverlastingBugstopper]

confirmation that this works with the trevor/update-tags-ref branch on supergraph-demo:

$ cargo xtask integration-test --branch trevor/update-tag-defs
    Finished dev [unoptimized + debuginfo] target(s) in 0.08s
     Running `target/debug/xtask integration-test --branch trevor/update-tag-defs`
info: running `cargo build --target x86_64-unknown-linux-gnu` in `/home/avery/work/rover`
info: successfully compiled to `/home/avery/work/rover/target/x86_64-unknown-linux-gnu/debug/rover`
info: running `git clone https://github.com/apollographql/supergraph-demo --branch trevor/update-tag-defs` in `/tmp/.tmpbdZhRE`
info: running `make ci` in `/tmp/.tmpbdZhRE/supergraph-demo`
info: successfully ran supergraph-demo with a local binary.
Success!

[EverlastingBugstopper]

Labelled this as blocked until we resolve the @tag repeatable breaking change that could be solved by this PR.