Deprecate service's management and `exec` commands

Proposed in https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/issues/1195#note_38777479. With upcoming %10.0 release we should consider to deprecate:

• gitlab-runner exec which is today hard to maintain, and without serious redesign it is effectively not working as expected.
• gitlab-runner (start|stop|install|uninstall|status) which is effective service management. We ask to run these commands on Linux/Windows/OSX, but maybe it would be better to refer to external tools instead and make runner a bare executable that relies on prepared configurations? (this one is also related with #2709 (closed))

These features should be marked as deprecated in %10.0 and removed in one of future releases.

Actions

Prerequisite: #2709 (closed)

• add an entry in the release post that deprecates gitlab-runner exec
• add an entry in the release post that deprecates gitlab-runner (start|stop|restart|install|uninstall|status)