Add ^15.0.0-rc.2 to peerDependencies range for graphql.

[abernix]

The upcoming graphql@15 is in a release-candidate stage right now. To further prepare, this PR expands the peerDependencies range of Apollo Server packages to allow those who wish to try the latest release candidate version (graphql@15.0.0-rc.2, at the time of this writing), prior to its finalization into a latest release.

This follows up #3712 which made the preliminarily necessary changes to accommodate various deprecations which it brings.

This PR is not meant to indicate that all Apollo Tooling is ready for what graphql@15 offers (e.g., as a notable addition, interfaces implementing interfaces), but should be the first step to ensuring that we can start testing out that process, since much of our own infrastructure relies on Apollo Server itself. Expect to see additional PRs as various avenues need to be addressed!

For now, we'll target this to the next release (currently v2.11.0) and cut an alpha release for users to try.

Two things worth noting:

• Peer dependencies are not usually strictly enforced, so this was possible before this commit, but this reduces the number of unmet peer dependency warning which are surfaced when using graphql@15 with Apollo Server.
• There are still a number of other peer dependency warnings which crop up. Those dependencies all exist outside of Apollo Server, and I'm not sure if it's worth chasing all of those down until the final v15 release, particularly since they are just peer dependency warnings.

[abernix]

Merged into #3763

[tvvignesh]

@abernix Tried upgrading graphql and installed the latest apollo-server-express package as well. Got this.

npm WARN apollo-graphql@0.4.1 requires a peer of graphql@^14.2.1 but none is installed. You must install peer dependencies yourself.
npm WARN apollo-link@1.2.13 requires a peer of graphql@^0.11.3 || ^0.12.3 || ^0.13.0 || ^14.0.0 but none is installed. You must install peer dependencies yourself.
npm WARN apollo-utilities@1.3.3 requires a peer of graphql@^0.11.0 || ^0.12.0 || ^0.13.0 || ^14.0.0 but none is installed. You must install peer dependencies yourself.
npm WARN graphql-tag@2.10.3 requires a peer of graphql@^0.9.0 || ^0.10.0 || ^0.11.0 || ^0.12.0 || ^0.13.0 || ^14.0.0 but none is installed. You must install peer dependencies yourself.
npm WARN graphql-tools@4.0.7 requires a peer of graphql@^0.13.0 || ^14.0.0 but none is installed. You must install peer dependencies yourself.
npm WARN graphql-upload@8.1.0 requires a peer of graphql@0.13.1 - 14 but none is installed. You must install peer dependencies yourself.
npm WARN subscriptions-transport-ws@0.9.16 requires a peer of graphql@^0.10.0 || ^0.11.0 || ^0.12.0 || ^0.13.1 || ^14.0.2 but none is installed. You must install peer dependencies yourself.
npm WARN graphql-subscriptions@1.1.0 requires a peer of graphql@^0.10.5 || ^0.11.3 || ^0.12.0 || ^0.13.0 || ^14.0.0 but none is installed. You must install peer dependencies yourself.

So, is it ok to upgrade to v15?

[nikosantis]

I have this problem with the peer dependencies
├─┬ apollo-server@2.13.1 │ └─┬ apollo-server-core@2.13.1 │ └─┬ @types/graphql-upload@8.0.3 │ └── graphql@14.6.0 ├── UNMET PEER DEPENDENCY graphql@15.0.0 └─┬ neo4j-graphql-js@2.14.0 └── UNMET PEER DEPENDENCY graphql@14.6.0