docs: set unicodeEmoji to false if using MySQL/MariaDB on BitBucket Server (#9185)

docs/usage/self-hosting.md

@@ -232,6 +232,8 @@ Configure it as `password` in your `config.js` file, or in environment variable
 Also configure the `username` for your bot account too, if you decided not to name it `@renovate-bot`.
 Don't forget to configure `platform=bitbucket-server` somewhere in config.
 
+If you use MySQL or MariaDB you must set `unicodeEmoji` to `false` in the bot config (`RENOVATE_CONFIG_FILE`) to prevent issues with emojis.
+
 ### Azure DevOps
 
 First, [create a personal access token](https://docs.microsoft.com/en-us/azure/devops/integrate/get-started/authentication/pats) for the bot account.