Add charset handling to backup/restore doc (#11062)

Co-authored-by: guillep2k <18600385+guillep2k@users.noreply.github.com>

docs/content/doc/usage/backup-and-restore.en-us.md

@@ -75,7 +75,7 @@ mv custom/conf/app.ini /etc/gitea/conf/app.ini # or mv app.ini /etc/gitea/conf/a
 unzip gitea-repo.zip
 mv gitea-repo/* /var/lib/gitea/repositories/
 chown -R gitea:gitea /etc/gitea/conf/app.ini /var/lib/gitea/repositories/
-mysql -u$USER -p$PASS $DATABASE <gitea-db.sql
+mysql --default-character-set=utf8mb4 -u$USER -p$PASS $DATABASE <gitea-db.sql
 # or  sqlite3 $DATABASE_PATH <gitea-db.sql
 service gitea restart
 ```

docs/content/doc/usage/backup-and-restore.zh-cn.md

@@ -54,7 +54,7 @@ mv custom/conf/app.ini /etc/gitea/conf/app.ini
 unzip gitea-repo.zip
 mv gitea-repo/* /var/lib/gitea/repositories/
 chown -R gitea:gitea /etc/gitea/conf/app.ini /var/lib/gitea/repositories/
-mysql -u$USER -p$PASS $DATABASE <gitea-db.sql
+mysql --default-character-set=utf8mb4 -u$USER -p$PASS $DATABASE <gitea-db.sql
 # or  sqlite3 $DATABASE_PATH <gitea-db.sql
 service gitea restart
 ```