fix: removing 'generated' folder from serverpod upgrade template

Closes: #2155

Pre-launch Checklist

[x] I read the Contribute page and followed the process outlined there for submitting PRs.
[x] This update contains only one single feature or bug fix and nothing else. (If you are submitting multiple fixes, please make multiple PRs.)
[x] I read and followed the Dart Style Guide and formatted the code with dart format.
[x] I listed at least one issue that this PR fixes in the description above.
[x] I updated/added relevant documentation (doc comments with ///), and made sure that the documentation follows the same style as other Serverpod documentation. I checked spelling and grammar.
[x] I added new tests to check the change I am making.
[x] All existing and new tests are passing.
[x] Any breaking changes are documented below.

Removing 'generated' folder from 'projectname_server_upgrade' template project.