[doc] Update the way to modify Android minSdkVersion in README

[millerf]

Flutter v2.8+ uses android/local.properties

See https://docs.flutter.dev/deployment/android#updating-the-apps-version-number (not really, but well...)

Pre-launch Checklist

• [ ] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [ ] I read the Tree Hygiene wiki page, which explains my responsibilities.
• [ ] I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/plugins repo does use dart format.)
• [x] I signed the CLA.
• [ ] The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• [ ] I listed at least one issue that this PR fixes in the description above.
• [ ] I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• [ ] I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style.
• [x] I updated/added relevant documentation (doc comments with ///).
• [x] I added new tests to check the change I am making, or this PR is test-exempt.
• [x] All existing and new tests are passing.

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[google-cla[bot]]

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[millerf]

Sorry, I am not used to this repo. Not sure of which branch to merge against

[stuartmorgan]

Thanks for the submission!

In the future, please do not delete the checklist that is in the PR template; it is there for a reason. This PR is missing required elements described in the checklist (I’ve restored it to the PR description), which need to be addressed before it moves forward with review.

I am marking the PR as a Draft. Please review the checklist, updating the PR as appropriate, and when that’s complete please feel free to mark the PR as ready for review.

[millerf]

In the future, please do not delete the checklist that is in the PR template; it is there for a reason.

Sorry about that. I'm used to unecessary lists. I'll fill the list later.

Thanks!

[Hixie]

@millerf Thanks for your contribution! Is this PR still on your radar?

[millerf]

@millerf Thanks for your contribution! Is this PR still on your radar?

Sorry I totally forgot about it. I can have a look tonight...

[stuartmorgan]

Since this is marked as a draft and hasn't been updated in several months I'm going to close it to clean out our review queue. Please don't hesitate to submit a new PR if you decide to revisit this. Thanks!