Update links from master to main for solidus_stripe repo

[waiting-for-dev]

Summary

Although GH would automatically take care of the redirection, it's better to just use the new name.

We can merge this PR just after renaming the branch.

Checklist

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[codecov]

Codecov Report

Merging #294 (dfe9db5) into master (8a1abc5) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #294   +/-   ##
=======================================
  Coverage   99.63%   99.63%           
=======================================
  Files          30       30           
  Lines         547      547           
=======================================
  Hits          545      545           
  Misses          2        2           

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