Fixed wording, typos, license notice & linked to migration guide

[fthobe]

Summary

Following changes have been made:

• given that the license is identical to parent repository, licensed under was changed
• integrated link to migration guide
• fixed some wording to give a more positive drive
• fixed some typos

Concerns (but doesn't fully fix yet) #6095

Checklist

Check out our PR guidelines for more details.

The following are mandatory for all PRs:

• I agree that my PR will be published under the same license as Solidus.
• I have written a thorough PR description.
• I have kept my commits small and atomic.
• I have used clear, explanatory commit messages.

The following are not always needed:

• 📖 I have updated the README to account for my changes.
• 📑 I have documented new code with YARD.
• 🛣️ I have opened a PR to update the guides.
• ✅ I have added automated tests to cover my changes.
• 📸 I have attached screenshots to demo visual changes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.70%. Comparing base (b367f99) to head (4f2c051).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6106   +/-   ##
=======================================
  Coverage   88.70%   88.70%           
=======================================
  Files         829      829           
  Lines       17992    17992           
=======================================
  Hits        15959    15959           
  Misses       2033     2033           

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