docs: add unsupported offline objects to migration guide

[mtrezza]

New Pull Request Checklist

• I am not disclosing a vulnerability.
• I am creating this PR in reference to an issue.

Issue Description

Offline storage is not supported by SDK, not documented in migration guide.

Related issue: #n/a

Approach

Add to migration guide.

TODOs before merging

n/a

[parse-github-assistant]

Thanks for opening this pull request!

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[codecov]

Codecov Report

Base: 90.36% // Head: 87.73% // Decreases project coverage by -2.62% ⚠️

Coverage data is based on head (9b71ed0) compared to base (8ec12ca).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #428      +/-   ##
==========================================
- Coverage   90.36%   87.73%   -2.63%     
==========================================
  Files         161      161              
  Lines       16267    16264       -3     
==========================================
- Hits        14699    14269     -430     
- Misses       1568     1995     +427     

Impacted Files	Coverage Δ
Sources/ParseSwift/Objects/ParseObject.swift	73.96% <0.00%> (-16.84%)	⬇️
Sources/ParseSwift/Objects/ParseInstallation.swift	74.44% <0.00%> (-11.97%)	⬇️
Sources/ParseSwift/Objects/ParseUser.swift	77.62% <0.00%> (-10.52%)	⬇️
...urces/ParseSwift/API/ParseURLSessionDelegate.swift	100.00% <0.00%> (ø)
Sources/ParseSwift/Coding/ParseEncoder.swift	78.14% <0.00%> (+0.45%)	⬆️

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