Replace "Login as guest" with "Try the app first" on login page

[turt2live]

Fixes element-hq/element-web#5875

[lukebarnard1]

I wonder what @lampholder thinks of this wording

[lampholder]

Ultimately, we will consider the wording on the registration/login page as part of a more holistic understanding of how users come to Riot, what they're trying to achieve, and how they should flow through the early stages.

As a small slice of that, I'd like to think that users who have arrived at the registration/login screen are there for one of three reasons:

1. they're already a user, come to login
2. they've come to try out Riot the technology/product
3. they know a community or person with which they want to engage uses Riot, and they want to reach them

If it's 1 or 3 then they're likely going to breeze past any 'try it first' option and go straight to logging in/registering.

If it's 2 then having the ability to poke around the app without creating an account is very appealing. The wording 'Try the app first' is pretty clear when presented adjacent to the alternative option 'Create an account' so I think I'm happy with it :)