Reduce request memory to 10MB.

This is based on a sample of 10.000 requests after launching on 
Platform.sh:

platform log --lines 10000 php.access | awk '{print $6}' | sort -n | uniq -c
Reading log file [email protected]:/var/log/php.access.log
9598 2048
 401 4096
   1 6144

We might even be able to go lower but 10MB is the minimum value on
Platform.sh.
https://docs.platform.sh/languages/php/fpm.html#2-adjust-the-maximum-number-of-php-fpm-workers

.platform.app.yaml

@@ -6,7 +6,10 @@ type: 'php:7.4'
 
 runtime:
   sizing_hints:
-    request_memory: 20
+    # A lean stack means that application memory usage is low.
+    # This allows us to have many workers which is useful as most time is spent
+    # waiting for token authorization.
+    request_memory: 10
 
 # The build-time dependencies of the app.
 dependencies: