Improve no editor detected state on conflicts modal

Right now, if we detect you don't have an editor configured, we show you this, as part of https://github.com/desktop/desktop/pull/6109

We've received feedback (in #6216 and elsewhere) that this is unclear. Based on metrics we receive through #5431, we're considering one these options as an improvement:

|Disabled button state|Open in default program|
|---|---|
|This option might encourage users to configure an editor if they haven't set theirs yet|This option would be a nicer experience for users with an editor that isn't supported|

We can use metrics to help inform this decision, but also very open to hear thoughts here!