Default soft limit for open files is 1024

## Steps to reproduce
1. connect to the running mongod server using mongosh (credentials may be needed, get these from juju unit data, etc)
2. Message pops up warning:
```
2023-08-28T05:11:08.183+00:00: Using the XFS filesystem is strongly recommended with the WiredTiger storage engine. See http://dochub.mongodb.org/core/prodnotes-filesystem
2023-08-28T05:11:12.730+00:00: Soft rlimits for open file descriptors too low
2023-08-28T05:11:12.730+00:00: currentValue: 1024
2023-08-28T05:11:12.730+00:00: recommendedMinimum: 64000
```
Adjust /etc/security/limits.conf to increase this to 64000. I just used "* soft nofile 64000" for now, but it would probably be better to restrict this further. This resulted in any other processes showing 64000 as the new soft limit, but when I restart the mongod service or reboot the instance, it doesn't increase this for mongod running in the snap. You can also confirm this by getting the pid of the mongod process and running `cat /proc//limits`

## Expected behavior
MongoDB recommends a minimum of 64000 for the NOFILES soft limit (num open files). See http://dochub.mongodb.org/core/prodnotes-filesystem
This might need further configuration - but certainly keeping it as low as 1024 seems to have known issues with causing crashes for many people. A more appropriate default for the charm should be used.

## Actual behavior
Default NOFILES seems to be 1024

## Versions
5/edge (rev 126)

Operating system:
20.04.6

Juju CLI: 2.9.44-ubuntu-amd64

Juju agent: 2.9.44

Charm revision: 126

LXD: 4.0.9

## Log output
Juju debug log:
(not relevant here)

## Additional context
Workaround is possible by editing /etc/systemd/system/snap.charmed-mongodb.mongod.service and adding `LimitNOFILE=64000` in the Service section. Then run `systemctl daemon-reload` and restart the service. After doing this, I can confirm that the open files limit is increased and the error goes away.

Start by inspecting the generated /etc/systemd/system/snap.charmed-mongodb.mongod.service and the mongod process limits via /proc//limits. Confirm how the snap service receives its NOFILES limit and verify completion when the effective soft limit is at least 64000 and the MongoDB warning no longer appears.

Written by the indexing model from the issue text.