Link to migration guide tempalte from pull request template (#58027)

https://dart.googlesource.com/external/github.com/flutter/flutter/+/4945a92fa6b2cd950c8cffbc4a4b0010b2163c07
1 file changed
tree: 3b064dd117e47e6400506a88e3ce5fb13665f802
  1. commits.json