[TACACS] Fix TACACS config revert to old config when device reboot issue. (sonic-net#22342)

Fix TACACS config revert to old config when device reboot issue.

#### Why I did it
Fix following bug:

1. When SONiC OS upgrade, old TACACS config will save to /etc/sonic/old_config/tacacs.json
2. After device reboot, TACACS config service (https://github.com/sonic-net/sonic-buildimage/blob/master/files/build_templates/tacacs-config.service) will restore TACACS config from /etc/sonic/old_config/tacacs.json, but this file will keep no change after restore TACACS config.
3. If TACACS service changed by user, because of #2, if device reboot again, the TACACS config been reverted back to old config in /etc/sonic/old_config/tacacs.json

Note: the TACACS config does not revert immediately after reboot, it will delay 5min 30sec:
https://github.com/sonic-net/sonic-buildimage/blob/master/files/build_templates/tacacs-config.timer

##### Work item tracking
- Microsoft ADO **(number only)**:32338799

#### How I did it
Move /etc/sonic/old_config/tacacs.json to /etc/sonic/old_config/tacacs.json_backup

#### How to verify it
Pass all test case.
Manually verify with following steps:

admin@vlab-01:~$ show tacacs
TACPLUS global auth_type login
TACPLUS global timeout 5 (default)
TACPLUS global passkey testing123

TACPLUS_SERVER address 10.250.0.102
               priority 1
               tcp_port 49

admin@vlab-01:~$ echo '
> {
>     "TACPLUS": {"global": { "auth_type": "login", "passkey": "12345" } }
> }' > /etc/sonic/old_config/tacacs.json
admin@vlab-01:~$ cat /etc/sonic/old_config/tacacs.json

{
    "TACPLUS": {"global": { "auth_type": "login", "passkey": "12345" } }
}

// then reboot device and wait for 6 minutes, because the TACACS config service will delay 5min 30sec after reboot:
https://github.com/sonic-net/sonic-buildimage/blob/master/files/build_templates/tacacs-config.timer

admin@vlab-01:~$ ls /etc/sonic/old_config/tacacs.json
ls: cannot access '/etc/sonic/old_config/tacacs.json': No such file or directory
admin@vlab-01:~$ show tacacs
TACPLUS global auth_type login
TACPLUS global timeout 5 (default)
TACPLUS global passkey 12345

TACPLUS_SERVER address 10.250.0.102
               priority 1
               tcp_port 49

#### Description for the changelog
Fix TACACS config revert to old config when device reboot issue.

Author: liuh-80

files/image_config/config-setup/config-setup

@@ -139,6 +139,9 @@ apply_tacacs()
         sonic-cfggen -j /etc/sonic/old_config/${TACACS_JSON_BACKUP} --write-to-db
         echo "Applied tacacs json to restore tacacs credentials"
         config save -y
+
+        # Change tacacs config file name, so tacacs config will not be revert when device reboot.
+        mv /etc/sonic/old_config/${TACACS_JSON_BACKUP} /etc/sonic/old_config/${TACACS_JSON_BACKUP}_backup
     else
         echo "Missing tacacs json to restore tacacs credentials"
     fi