fix: rename "phone" courier channel to "sms"

[jonas-jonas]

Related issue(s)

Checklist

• I have read the contributing guidelines.
• I have referenced an issue containing the design document if my change
  introduces a new feature.
• I am following the
  contributing code guidelines.
• I have read the security policy.
• I confirm that this pull request does not address a security
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• I have added tests that prove my fix is effective or that my feature
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Further Comments

[codecov]

Codecov Report

Attention: 17 lines in your changes are missing coverage. Please review.

Comparison is base (699e5d5) 78.34% compared to head (7e22049) 78.33%.

Files	Patch %	Lines
identity/extension_verification.go	80.64%	3 Missing and 3 partials ⚠️
courier/http_channel.go	20.00%	4 Missing ⚠️
request/builder.go	0.00%	4 Missing ⚠️
selfservice/strategy/code/code_sender.go	50.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3680      +/-   ##
==========================================
- Coverage   78.34%   78.33%   -0.01%     
==========================================
  Files         346      346              
  Lines       23548    23551       +3     
==========================================
  Hits        18448    18448              
+ Misses       3711     3710       -1     
- Partials     1389     1393       +4     

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[zepatrik]

Just some minor things. For recovery we'll do the same, but we don't yet support SMS recovery, right?