Add help text to the New group name field when signing up for a trial

The following discussion from !120536 (merged) should be addressed:

• @gdoyle started a discussion: (+1 comment)

  suggestion (non-blocking): You could include the requirements for a valid input for New Group Name here as help text. Something like: Must contain only letters, digits, emojis, '_', '.', dash, space, and parenthesis. Must start with a letter, digit, emoji, or '_'.

You can add validation directly to the New group name field instead of making it an error post-submission. This way, you can provide the messaging before the user has to go through submit flow.

• engineering note: I don't think we can do this fully, but perhaps if we took pieces from here we could add the ability to do that here. There is room here to not do this too if it takes this issue too far out of scope. The core ask here is that we attached group/namespace related errors to the new_group_name field instead of the generic top banner. We should still say something generically at the top about there being errors though, just list these specific ones on the input instead.