in ftp cache pruning, avoid changing the size of a dict while iterating over it (closes #21463)

Patch by Skyler Leigh Amador.
diff --git a/Misc/ACKS b/Misc/ACKS
index 571f1a6..344a29b 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -32,6 +32,7 @@
 Ray Allen
 Billy G. Allie
 Kevin Altis
+Skyler Leigh Amador
 Joe Amenta
 A. Amoroso
 Mark Anacker