Improve messaging about authentication

[jesse-ross]

This improves the messages about the changes in authentication which are given when people try to run the old authenticate_sb().

• The message displayed whenever that function is run now states that SB has switched over to requiring MFA, and mentions that role accounts are an exception can still use password-based login.
• The spurious "Unable to authenticate to legacy SB." warning is now removed. I think we can rely on the message when the function is called, or on people seeing that is_logged_in() == FALSE

Closes #323.

[codecov-commenter]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (9f3ab9d) 39.62% compared to head (e06015c) 39.48%.

❗ Current head e06015c differs from pull request most recent head bbd31a7. Consider uploading reports for the commit bbd31a7 to get more accurate results

Files	Patch %	Lines
R/authenticate_sb.R	0.00%	5 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #324      +/-   ##
==========================================
- Coverage   39.62%   39.48%   -0.14%     
==========================================
  Files          23       23              
  Lines         853      856       +3     
==========================================
  Hits          338      338              
- Misses        515      518       +3     

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