Fix README: cert-migration note, verify example, and typo

[AndreyVMarkelov]

Docs-only cleanup ahead of the next release.

• Reword the v7.0.0 requirement to past tense now that the January 2026 certificate-migration deadline has passed.
• Show the ContentHasher example verifying a downloaded file against its ContentHash metadata (addresses how to verify downloaded file? #34) and add the required using directive.
• Fix the 'offical' -> 'official' typo.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.60%. Comparing base (3cc79e1) to head (3a97e64).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #386   +/-   ##
=======================================
  Coverage   61.60%   61.60%           
=======================================
  Files          17       17           
  Lines        1654     1654           
  Branches      155      155           
=======================================
  Hits         1019     1019           
  Misses        565      565           
  Partials       70       70           

Flag	Coverage Δ
integration	57.19% <ø> (ø)
unit	33.91% <ø> (ø)

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