feat: Add pre-upgrade warning for APIC 6.2.1 that downgrade is not supported to pre-6.2.1 version.

[saket1999]

Starting from 6.2.1 release, policy based downgrade will not be supported to pre-6.2.1 version, for this, a warning is being added to let customers know that downgrade is not supported.

pytest output:

pytest tests/apic_621_upgrade_warning/test_apic_621_upgrade_warning.py 
============================================================================================= test session starts =============================================================================================
platform linux -- Python 3.8.10, pytest-7.2.0, pluggy-1.5.0
rootdir: /data/ssd/ACI-Pre-Upgrade-Validation-Script, configfile: pytest.ini
collected 8 items                                                                                                                                                                                             

tests/apic_621_upgrade_warning/test_apic_621_upgrade_warning.py::test_logic[None-None-MANUAL CHECK REQUIRED] 
----------------------------------------------------------------------------------------------- live log setup ------------------------------------------------------------------------------------------------
[09:38:42.835 INFO               initialize:5373] Cleaning up previous run files in preupgrade_validator_logs/
[09:38:42.835 INFO               initialize:5375] Creating directories preupgrade_validator_logs/ and preupgrade_validator_logs/json_results/
PASSED                                                                                                                                                                                                  [ 12%]
tests/apic_621_upgrade_warning/test_apic_621_upgrade_warning.py::test_logic[4.2(1b)-None-MANUAL CHECK REQUIRED] PASSED                                                                                  [ 25%]
tests/apic_621_upgrade_warning/test_apic_621_upgrade_warning.py::test_logic[None-5.2(2a)-MANUAL CHECK REQUIRED] PASSED                                                                                  [ 37%]
tests/apic_621_upgrade_warning/test_apic_621_upgrade_warning.py::test_logic[5.2(2a)-6.1(4a)-PASS] PASSED                                                                                                [ 50%]
tests/apic_621_upgrade_warning/test_apic_621_upgrade_warning.py::test_logic[6.1(3a)-6.1(4c)-PASS] PASSED                                                                                                [ 62%]
tests/apic_621_upgrade_warning/test_apic_621_upgrade_warning.py::test_logic[6.1(3a)-6.2(1a)-MANUAL CHECK REQUIRED] PASSED                                                                               [ 75%]
tests/apic_621_upgrade_warning/test_apic_621_upgrade_warning.py::test_logic[6.1(3a)-6.2(2a)-MANUAL CHECK REQUIRED] PASSED                                                                               [ 87%]
tests/apic_621_upgrade_warning/test_apic_621_upgrade_warning.py::test_logic[6.2(1a)-6.2(2c)-PASS] PASSED                                                                                                [100%]

============================================================================================== 8 passed in 0.02s ==============================================================================================