feat: support MikroORM v6

[ssut]

BREAKING CHANGE:

MikroORM v6 and Nest.js v10 are now required. Also Node.js 18.12 is required.

[B4nan]

fwiw, v6 is already supported, there are no required changes here, the peer dependency range should already allow stable v6 too (works fine with the realworld example)

[B4nan]

Note for myself so I don't forget, we also need to rename the injection tokens for repositories (#106) for v6.

[tsangste]

something that needs to be considered for this PR is that MikroORM via the re-export changes could be no longer viable as a DI token due to it potentially being another type e.g. MikroORM export from @mikro-orm/postgresql is actually PostgreSqlMikroORM.

import { MikroORM } from '@mikro-orm/postgresql'

@Injectable()
export class TestService {
  constructor(private readonly orm: MikroORM) {}
}

It would fail with the error Please make sure that the argument PostgreSqlMikroORM at index [0] is available in the RootTestModule context.

You can work around this via one of the following:

1. Keep MikroORM type import from @mikro-orm/core
2. Give MikroORM a context name and use @InjectMikroORM() decorator for injection