Update VPN client to 0.21.6

[adguard-octopass]

A new VPN client version 0.21.6 has been released. Please integrate it.

Changelog: https://github.com/AdGuardSoftwareLimited/vpn-client/releases/tag/v0.21.6

Changelog

Changed

• Breaking: [Native] ag::vpn::WebRequestsManager now has a two-phase lifecycle: shutdown()
  MUST be called before the manager is destroyed, and the destructor aborts if it was not.
• Update vpn-libs to 1.1.5-rc.3

Fixed

• Fixed a crash when the VPN was stopped while a backend request was still in flight.