Enable Style/For rubocop cop

Do not use for, unless you know exactly why.

See #17478

.rubocop.yml

@@ -247,7 +247,7 @@ Style/FlipFlop:
 
 # Checks use of for or each in multiline loops.
 Style/For:
-  Enabled: false
+  Enabled: true
 
 # Enforce the use of Kernel#sprintf, Kernel#format or String#%.
 Style/FormatString:

app/models/network/graph.rb

@@ -205,7 +205,7 @@ module Network
       # Visit branching chains
       leaves.each do |l|
         parents = l.parents(@map).select{|p| p.space.zero?}
-        for p in parents
+        parents.each do |p|
           place_chain(p, l.time)
         end
       end
@@ -223,7 +223,7 @@ module Network
     end
 
     def mark_reserved(time_range, space)
-      for day in time_range
+      time_range.each do |day|
         @reserved[day].push(space)
       end
     end
@@ -232,7 +232,7 @@ module Network
       space_default ||= space_base
 
       reserved = []
-      for day in time_range
+      time_range.each do |day|
         reserved.push(*@reserved[day])
       end
       reserved.uniq!