nixos/jitsi-meet: fix systemd protection flag warning

Fixes systemd warning:

> /etc/systemd/system/jitsi-excalidraw.service:26: Failed to parse ProtectProc=true, ignoring: Invalid argument

when Jitsi-Meet Excalidraw Backend is enabled. `ProtectProc=true` is not a thing according to man pages, `ProtectProc=noaccess` is the most restrictive setting and likely what was originally intentioned.

Changed files
+1 -1
nixos
modules
services
web-apps
+1 -1
nixos/modules/services/web-apps/jitsi-meet.nix
···
ProtectSystem = "strict";
ProtectClock = true;
ProtectHome = true;
-
ProtectProc = true;
+
ProtectProc = "noaccess";
ProtectKernelLogs = true;
PrivateTmp = true;
PrivateDevices = true;