[db_migrator] Fix the broken version chain

[vivekrnv]

Why I did

Warm Reboot from 202205 -> latest labels is failing because of an extra version that was added to 202205 and that was missing in later branches.

How I did it

Add a placeholder for version 3_0_7 in latest labels

How to verify it

WR from 202205 to 202305 and try running db_migrator

root@r-anaconda-simx-139:/home/admin# db_migrator.py -o migrate
Traceback (most recent call last):
  File "/usr/local/bin/db_migrator.py", line 1133, in main
    result = getattr(dbmgtr, operation)()
  File "/usr/local/bin/db_migrator.py", line 1085, in migrate
    next_version = getattr(self, version)()
AttributeError: 'DBMigrator' object has no attribute 'version_3_0_7'
'DBMigrator' object has no attribute 'version_3_0_7'
usage: db_migrator.py [-h] [-o operation migrate, set_version, get_version] [-s unix socket] [-n asic namespace]

optional arguments:
  -h, --help            show this help message and exit
  -o operation (migrate, set_version, get_version)
                        operation to perform [default: get_version]
  -s unix socket        the unix socket that the desired database listens on
  -n asic namespace     The asic namespace whose DB instance we need to connect

With this change:

root@r-anaconda-simx-139:/home/admin# db_migrator.py -o migrate
root@r-anaconda-simx-139:/home/admin# redis-cli -n 4 hgetall "VERSIONS|DATABASE"
1) "VERSION"
2) "version_4_0_3"

Previous command output (if the output of a command-line utility has changed)

New command output (if the output of a command-line utility has changed)

[dgsudharsan]

@StormLiangMS Can you please cherry-pick this bug fix to 202305? This is required for doing the upgrade tests

[StormLiangMS]

ADO:25592749