Previously, UpgradeApplicator would open build.gradle
using open options that left the fields existing content intact. It
would then write the new content at the beginning of the file. If
the new content was n bytes shorter than the existing content, this
would leave n bytes of the existing content at the end of the file.

This commit updates UpgradeApplicator to truncate the existing file
when it opens it. This ensures that the existing content is
completely replaced by the new content, irrespective of their lengths.

Closes gh-25256