feat: change code length to 6 numbers

[jonas-jonas]

Related issue(s)

Checklist

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  introduces a new feature.
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Further Comments

[codecov]

Codecov Report

Merging #2894 (07fb725) into master (02a92b4) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 07fb725 differs from pull request most recent head 6926ed7. Consider uploading reports for the commit 6926ed7 to get more accurate results

@@            Coverage Diff             @@
##           master    #2894      +/-   ##
==========================================
- Coverage   76.11%   76.11%   -0.01%     
==========================================
  Files         308      308              
  Lines       18873    18870       -3     
==========================================
- Hits        14366    14363       -3     
  Misses       3389     3389              
  Partials     1118     1118              

Impacted Files	Coverage Δ
selfservice/strategy/code/strategy.go	100.00% <100.00%> (ø)
selfservice/strategy/code/strategy_recovery.go	70.19% <100.00%> (ø)
session/handler.go	67.81% <0.00%> (-0.14%)	⬇️
driver/config/config.go	82.72% <0.00%> (-0.06%)	⬇️

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[jonas-jonas]

This was originally a suggestion by @zepatrik, but IMO proved impractical, as the error message from this is less than helpful ("does not match pattern ^\d{8}$") and in the end, we "validate" this field by checking if the code is correct, and if someone enters a code that is not 8 (after this PR 6) characters long, then that code is invalid anyway.