[#68880956] Use upstream openconnect module

Switch to the open source module that this code later evolved into.

Some key points:

- The vpnc dependency is now bundled into the openconnect module itself.
- This version logs to syslog so we no longer need to append our own
  timestamps to the logs.
- Thus I've removed `gawk` from `system_packages` which is no longer needed
  on new jumpbox machines.
- Fixes a bug whereby the daemon wasn't restarted on argument changes. I
  think this prevented the timestamp fix going out to some PP environments.
- I haven't included a version in `Puppetfile` because it is already pinned
  by `Puppetfile.lock`.

Depends on gds/pp-deployment#206 for new hiera key names.
Merge pull request #354 from alphagov/68880956-upstream_openconnect

[#68880956] Use upstream openconnect module