fix: registration post persist hooks should not be cancelable

[zepatrik]

Related issue(s)

When a request is canceled at the wrong point, it can lead to some hooks not being executed.
With this change we ignore context cancelation after the identity was created so that e.g. verification emails or webhooks will always be executed.

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
  vulnerability. If this pull request addresses a security vulnerability, I
  confirm that I got the approval (please contact
  [email protected]) from the maintainers to push
  the changes.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added or changed the documentation.

Further Comments

IMO this is expected behavior so no docs changes should be required.

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 78.58%. Comparing base (629d867) to head (3fc5a4e).
Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
selfservice/flow/registration/hook.go	80.00%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4148      +/-   ##
==========================================
- Coverage   78.61%   78.58%   -0.04%     
==========================================
  Files         378      379       +1     
  Lines       26936    26989      +53     
==========================================
+ Hits        21177    21208      +31     
- Misses       4147     4169      +22     
  Partials     1612     1612              

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