Update fallback stability requirements

Decrease the minimum number of fallbacks to 100.

Final fix to 20913.

changes/bug20913

@@ -1,7 +1,9 @@
   o Minor bugfixes (fallbacks):
     - Make sure fallback directory mirrors have the same address, port, and
       relay identity key for at least 30 days before they are selected.
-      Partial fix to 20913, bugfix on 0.2.8.1-alpha.
+      Fixes 20913, bugfix on 0.2.8.1-alpha.
     - Decrease the guard flag average required to be a fallback. This allows
       us to keep relays that have their guard flag removed when they restart.
-      Partial fix to 20913, bugfix on 0.2.8.1-alpha.
+      Fixes 20913, bugfix on 0.2.8.1-alpha.
+    - Decrease the minimum number of fallbacks to 100.
+      Fixes 20913, bugfix on 0.2.8.1-alpha.

scripts/maint/updateFallbackDirs.py

@@ -198,7 +198,7 @@ FALLBACK_PROPORTION_OF_GUARDS = None if OUTPUT_CANDIDATES else _FB_POG
 # Limit the number of fallbacks (eliminating lowest by advertised bandwidth)
 MAX_FALLBACK_COUNT = None if OUTPUT_CANDIDATES else 200
 # Emit a C #error if the number of fallbacks is less than expected
-MIN_FALLBACK_COUNT = 0 if OUTPUT_CANDIDATES else MAX_FALLBACK_COUNT*0.75
+MIN_FALLBACK_COUNT = 0 if OUTPUT_CANDIDATES else MAX_FALLBACK_COUNT*0.5
 
 # The maximum number of fallbacks on the same address, contact, or family
 # With 200 fallbacks, this means each operator can see 1% of client bootstraps