Please help me roll back gradle

I develop Android apps on Qt5/C++ on Linux workstation. I have made and published many apps for API 26 via gradle 2.2.3. To meet Google 64-bit compatibility requirements I experimented with latest Qt5 versions and in one project I changed settings to
All other projects contained lines for API 26. After that I returned to API 26 cause support of API 28 in Qt5 is not ready yet. But I have forgotten change lines above back to API 26. Then I started build this project - the Android Studio (it runs at background) suddenly and without my permission downloaded and installed gradle 3.x. With it all other projects appear broken - they cannot build APKs with bunch of errors related to Google Play services. They ignore requests to build for API 26 and try build for API 28. Now I do not have time to upgrade and solve all this issues - I only need to roll back entire system for development with gradle 2.2.3 for API 26. I restored all source files of first broken project from backup. The build.gradle in it file contains line:
classpath ‘’
and has lines

But application build stops with error:

FAILURE: Build failed with an exception.

  • What went wrong:
    A problem occurred configuring root project ‘android-build’.
    Failed to notify project evaluation listener.;)V

This did not appear when I worked before with gradle 2.2.3. Something changed while gradle 3.x installation but I do not know what. Please help me find what I should restore or remove. I must ASAP return to API 26 to make important changes in currently developed new project.