[runners.ssh.disable_strict_host_key_checking] default to false

In !3074 (merged), we added a configuration value [runners.ssh.disable_strict_host_key_checking] in the config.toml that controls whether or not to use strict host key checking with SSH.

Before 15.0, this value defaults to true. If it's not set, we disable strict host key checking (which is how it has been up to the change in !3074 (merged)).

Starting in 15.0, we want the default value of [runners.ssh.disable_strict_host_key_checking] to be false, so that it's required by default.