How to increase hard NOFILE limit in debian 11?

[Thomas Nyberg <twn+debian-user-list@thomasnyberg.com>]

Hello,

I'm running stock debian 11:

```
$ lsb_release -a
No LSB modules are available.
Distributor ID:	Debian
Description:	Debian GNU/Linux 11 (bullseye)
Release:	11
Codename:	bullseye
$ uname -r
5.10.0-19-cloud-amd64
```

I have the following default hard limit on open files:

```
$ ulimit -Hn
1048576
```

I would like to increase that (and not just for my bash process), but no 
matter what I try after searching online, I'm unable to change it. I've 
tried changing this in `/etc/security/limits.conf`, in 
`/etc/systemd/{system,user}.conf`, and I've tried using `prlimit` to 
directly change the limits for the process in question (receiving an 
error when doing so).

Does anyone here know how to increase the hard limit on open files for 
processes running debian 11? Thanks for any help!

Cheers,
Thomas