database server: unpin pysnmp-lextudio

Currently if you use 6.0.x of pysnmp-lextudio (required for the snmp moxa stats part of the database server) you get an error essage like this:

```
[Mon Mar 18 11:12:28 2024] Exception in thread Thread-1 (_update_snmp):

[Mon Mar 18 11:12:28 2024] Traceback (most recent call last):

[Mon Mar 18 11:12:28 2024] File "C:\Instrument\Apps\Python3\Lib\threading.py", line 1045, in _bootstrap_inner

[Mon Mar 18 11:12:28 2024] self.run()

[Mon Mar 18 11:12:28 2024] File "C:\Instrument\Apps\Python3\Lib\threading.py", line 982, in run

[Mon Mar 18 11:12:28 2024] self._target(*self._args, **self._kwargs)

[Mon Mar 18 11:12:28 2024] File "C:\Instrument\Apps\EPICS\ISIS\inst_servers\master\DatabaseServer\moxa_data.py", line 148, in _update_snmp

[Mon Mar 18 11:12:28 2024] mibmap = walk(ip_addr, SYSTEM_MIBS + PORT_MIBS)

[Mon Mar 18 11:12:28 2024] ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

[Mon Mar 18 11:12:28 2024] File "C:\Instrument\Apps\EPICS\ISIS\inst_servers\master\server_common\snmpWalker.py", line 23, in walk

[Mon Mar 18 11:12:28 2024] varBinds) in nextCmd(SnmpEngine(),

[Mon Mar 18 11:12:28 2024] ^^^^^^^^^^^^^^^^^^^^^

[Mon Mar 18 11:12:28 2024] File "C:\Instrument\Apps\Python3\Lib\site-packages\pysnmp\hlapi\asyncio\sync\cmdgen.py", line 271, in nextCmd

[Mon Mar 18 11:12:28 2024] loop = asyncio.get_event_loop()

[Mon Mar 18 11:12:28 2024] ^^^^^^^^^^^^^^^^^^^^^^^^

[Mon Mar 18 11:12:28 2024] File "C:\Instrument\Apps\Python3\Lib\asyncio\events.py", line 677, in get_event_loop

[Mon Mar 18 11:12:28 2024] raise RuntimeError('There is no current event loop in thread %r.'

[Mon Mar 18 11:12:28 2024] RuntimeError: There is no current event loop in thread 'Thread-1 (_update_snmp)'.
```

We should look and see if we can use v6 onwards as it's not a good idea to pin dependencies!

https://github.com/lextudio/pysnmp/issues/49 is tracking this

## Time in planning
1:25:50 4/4/24
also discussed briefly 02/05/2024

## Time in planning meeting (18/07/24)
03:05 Agreed to take with dependency updates ticket next time round as issue linked above now closed