ci: When models change, dump gql schema

[Yaminyam]

To easily identify when the graphql schema has changed, a dump is created when files in the models path are modified.

[achimnol]

• Let's generalize the workflow filename and the top-level name key to update-api-schema.
• Let's rename the job to graphql.

This will give a concise yet extensible check name "update-api-schema / graphql" where we could add "update-api-schema / rest" in the future.

• Let's also migrate the OpenAPI XML generator (./py -m ai.backend.manager.openapi) to mgr openapi show command.

[achimnol]

As of #1686, we should use mgr api dump-gql-schema instead of mgr gql show.
We also don't need to populate dummy manager.toml.

[Yaminyam]

• Let's also migrate the OpenAPI XML generator (./py -m ai.backend.manager.openapi) to mgr openapi show command.

As with graphql, it was decided to replace this with mgr api dump-openapi.

[achimnol]

Please resolve the merge conflict. I think the conflict is due to the formatting rule change in #1771, so you may git checkout --ours, check if there are any missing attributes added in the main branch, and reformat it again using pants fix and pants fmt. See #1771's description about the update guide.