Use FLOAT_TYPE for possibly large fields.

This does update the schema, but does not require large updates in the frontend, as opposed to Types::BigInt which would require string -> number casting. 
GraphQL-induced type casting should prevent type issues according to https://spec.graphql.org/June2018/#sec-Float

The values allowed in this type should suffice for the foreseeable future.

Also  updated the changelog to document this limitation.

app/graphql/types/project_statistics_type.rb

@@ -6,20 +6,20 @@ module Types
 
     authorize :read_statistics
 
-    field :commit_count, GraphQL::Types::BigInt, null: false,
+    field :commit_count, GraphQL::FLOAT_TYPE, null: false,
           description: 'Commit count of the project'
 
-    field :storage_size, GraphQL::Types::BigInt, null: false,
+    field :storage_size, GraphQL::FLOAT_TYPE, null: false,
           description: 'Storage size of the project'
-    field :repository_size, GraphQL::Types::BigInt, null: false,
+    field :repository_size, GraphQL::FLOAT_TYPE, null: false,
           description: 'Repository size of the project'
-    field :lfs_objects_size, GraphQL::Types::BigInt, null: false,
+    field :lfs_objects_size, GraphQL::FLOAT_TYPE, null: false,
           description: 'Large File Storage (LFS) object size of the project'
-    field :build_artifacts_size, GraphQL::Types::BigInt, null: false,
+    field :build_artifacts_size, GraphQL::FLOAT_TYPE, null: false,
           description: 'Build artifacts size of the project'
-    field :packages_size, GraphQL::Types::BigInt, null: false,
+    field :packages_size, GraphQL::FLOAT_TYPE, null: false,
           description: 'Packages size of the project'
-    field :wiki_size, GraphQL::Types::BigInt, null: true,
+    field :wiki_size, GraphQL::FLOAT_TYPE, null: true,
           description: 'Wiki size of the project'
   end
 end

app/graphql/types/root_storage_statistics_type.rb

@@ -6,11 +6,11 @@ module Types
 
     authorize :read_statistics
 
-    field :storage_size, GraphQL::Types::BigInt, null: false, description: 'The total storage in bytes'
-    field :repository_size, GraphQL::Types::BigInt, null: false, description: 'The Git repository size in bytes'
-    field :lfs_objects_size, GraphQL::Types::BigInt, null: false, description: 'The LFS objects size in bytes'
-    field :build_artifacts_size, GraphQL::Types::BigInt, null: false, description: 'The CI artifacts size in bytes'
-    field :packages_size, GraphQL::Types::BigInt, null: false, description: 'The packages size in bytes'
-    field :wiki_size, GraphQL::Types::BigInt, null: false, description: 'The wiki size in bytes'
+    field :storage_size, GraphQL::FLOAT_TYPE, null: false, description: 'The total storage in bytes'
+    field :repository_size, GraphQL::FLOAT_TYPE, null: false, description: 'The Git repository size in bytes'
+    field :lfs_objects_size, GraphQL::FLOAT_TYPE, null: false, description: 'The LFS objects size in bytes'
+    field :build_artifacts_size, GraphQL::FLOAT_TYPE, null: false, description: 'The CI artifacts size in bytes'
+    field :packages_size, GraphQL::FLOAT_TYPE, null: false, description: 'The packages size in bytes'
+    field :wiki_size, GraphQL::FLOAT_TYPE, null: false, description: 'The wiki size in bytes'
   end
 end

changelogs/unreleased/23131-upgrade-statistics-endpoints.yml

 ---
 title: >
-    #42671: Project and group storage statistics now support values larger than 4GiB
+    #42671: Project and group storage statistics now support values up to 8 PiB (up from 4GiB)
 merge_request: 23131
 author: Matthias van de Meent
 type: fixed