Removing migration guide (no longer relevant) (#763)

* Removing page

* Editing _meta.js

* Update _meta.js

---------

Co-authored-by: Benoît Rouleau <[email protected]>

Author: MichaelMacaulay

website/pages/en/cookbook/_meta.js

@@ -1,5 +1,4 @@
 export default {
-  'upgrading-a-subgraph': '',
   'subgraph-debug-forking': '',
   near: '',
   cosmos: '',