[MM-4819] Add MinimumHashtagLength config (#10248)

#### Summary
Add `MinimumHashtagLength` config

#### Ticket Link
#9156 
https://mattermost.atlassian.net/browse/MM-4819

#### Checklist
N/A

config/default.json

@@ -61,6 +61,7 @@
         "ExperimentalEnableAuthenticationTransfer": true,
         "TimeBetweenUserTypingUpdatesMilliseconds": 5000,
         "EnablePostSearch": true,
+        "MinimumHashtagLength": 3,
         "EnableUserTypingMessages": true,
         "EnableChannelViewedMessages": true,
         "EnableUserStatuses": true,