Enable the ESLint prefer-const rule globally (PR 11450 follow-up)

Please find additional details about the ESLint rule at https://eslint.org/docs/rules/prefer-const

With the recent introduction of Prettier this sort of mass enabling of ESLint rules becomes a lot easier, since the code will be automatically reformatted as necessary to account for e.g. changed line lengths.

Note that this patch is generated automatically, by using the ESLint `--fix` argument, and will thus require some additional clean-up (which is done separately).

web/.eslintrc

@@ -9,6 +9,5 @@
 
     // ECMAScript 6
     "no-var": "error",
-    "prefer-const": "error",
   },
 }