[docker-database] Fix Python3 issue (#7700)

#### Why I did it
To avoid the following error
```
Traceback (most recent call last):
  File "/usr/local/bin/flush_unused_database", line 10, in <module>
    if 'PONG' in output:
TypeError: a bytes-like object is required, not 'str'
```
`communicate` method returns the strings if streams were opened in text mode; otherwise, bytes.
In our case text arg  in Popen is not true and that means that `communicate` return the bytes
#### How I did it
Set `text=True` to get strings instead of bytes
#### How to verify it
run `/usr/local/bin/flush_unused_database` inside database container

dockers/docker-database/flush_unused_database

@@ -5,7 +5,7 @@ import subprocess
 import time
 
 while(True):
-    output = subprocess.Popen(['sonic-db-cli', 'PING'], stdout=subprocess.PIPE).communicate()[0]
+    output = subprocess.Popen(['sonic-db-cli', 'PING'], stdout=subprocess.PIPE, text=True).communicate()[0]
     if 'PONG' in output:
         break
     time.sleep(1)