Update SwiftPM mentions on pubspec reference page

[parlough]

• Remove the documentation for the disable-swift-package-manager which has been superseded by the enable-swift-package-manager field under config.
  • Keep an anchor with the same fragment so it directs existing links to the new field's documentation.
• Add a section explicitly covering the enable-swift-package-manager field.
• Add a version note explaining how SwiftPM was previously not enabled by default. Also reference our SwiftPM docs.

Staged: https://flutter-docs-prod--docs-pr13598-misc-swift-pm-pubspec-2ofp67zl.web.app/tools/pubspec#enable-swift-package-manager

[gemini-code-assist]

Code Review

This pull request updates the pubspec.md documentation to reflect changes in how Swift Package Manager (SPM) is configured. Specifically, it removes the deprecated top-level disable-swift-package-manager field and introduces the new enable-swift-package-manager field under the config section, detailing how to disable SPM and noting that this disable option will be removed in a future release. I have no feedback to provide as there are no review comments.

[flutter-website-bot]

Staged preview of the updated docs.flutter.dev site (updated for commit b372ec4):

https://flutter-docs-prod--docs-pr13598-misc-swift-pm-pubspec-2ofp67zl.web.app

[flutter-website-bot]

Staged preview of the updated flutter.dev site (updated for commit b372ec4):

https://flutter-dev-230821--www-pr13598-misc-swift-pm-pubspec-pbojoilb.web.app

[sfshaza2]

lgtm