Remove legacy keystone.extendGraphQLSchema()

[timleslie]

See #5025

This legacy method is no longer required, and removing it takes a large chunk of legacy code away too. The equivalent functionality is now provided by config.extendGraphqlSchema.

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🦋 Changeset detected

Latest commit: 154f95b

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 11 packages

Name	Type
@keystone-next/keystone-legacy	Major
@keystone-next/adapter-knex-legacy	Patch
@keystone-next/adapter-mongoose-legacy	Patch
@keystone-next/adapter-prisma-legacy	Patch
@keystone-next/test-utils-legacy	Patch
@keystone-next/cypress-project-access-control-legacy	Patch
@keystone-next/cypress-project-basic-legacy	Patch
@keystone-next/cypress-project-client-validation-legacy	Patch
@keystone-next/cypress-project-login-legacy	Patch
@keystone-next/benchmarks-legacy	Patch
@keystone-next/keystone	Patch

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