[mongoose] update to 7.21

[AenBleidd]

• Changes comply with the maintainer guide.
• SHA512s are updated for each updated download.
• The "supports" clause reflects platforms that may be fixed by this new version, or no changes were necessary.
• Any fixed CI baseline and CI feature baseline entries are removed from that file, or no entries needed to be changed.
• All patch files in the port are applied and succeed.
• The version database is fixed by rerunning ./vcpkg x-add-version --all and committing the result.
• Exactly one version is added in each modified versions file.

[Copilot]

Pull request overview

Updates the vcpkg port for cesanta/mongoose to the new upstream release 7.21, including the corresponding version database and baseline adjustments so consumers resolve to the updated port.

Changes:

• Bump ports/mongoose from 7.20 (port-version 1) to 7.21 (port-version 0).
• Update the port source hash for the new upstream ref.
• Add 7.21 to the version database and move the baseline to 7.21.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
versions/m-/mongoose.json	Adds the new 7.21 git-tree entry to the versions database.
versions/baseline.json	Advances the baseline mongoose version to 7.21 and resets port-version to 0.
ports/mongoose/vcpkg.json	Updates the port manifest version to 7.21 and drops the now-unneeded explicit port-version.
ports/mongoose/portfile.cmake	Updates the SHA512 to match the 7.21 upstream download.

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