Add stringio to manifest.yml

[ksss]

stringio is scheduled to be moved to stdlib, so it should be listed in manifest.yml.
stringio is already set up in csv, and no problems have been reported.
This change requires rbs v3.7 or higher, but since it has been more than 7 months since its release, we consider it to have become widely accepted.

Please let us know if you have any problems.

ref: ruby/rbs#1549

[github-actions]

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