Ugh, another dependency nightmare. Most platforms actually have backwards compatibility that lasts for more than a couple of years - Google Play's aggressive deprecation of old SDKs is a real maintenance headache. If they supported things for longer then things like this would happen far less.
The situation is Construct's mobile ad plugin relies on admob-plus-cordova. The latest stable version is v1.28.0 which uses com.google.android.gms:play-services-ads@20.6.0, the version Google Play now says has been sunset. We've been keeping an eye on updates to admob-plus-cordova for some time now, but the latest version is 2.0.0-alpha.18. The term "alpha" usually means "for early testing only and not ready for production use yet", so we've been waiting for a stable release before updating Construct. But Google Play have aggressively retired the old library version before this happened.
I'm aware of community-admob-plus, but long experience has shown that switching between different libraries with different maintainers can cause its own headaches with compatibility, maintenance, etc. So in my view the ideal situation is a stable update of admob-plus-cordova to be released. However we are out of time really so we may have to make a pragmatic decision. I will investigate our options and hopefully have something lined up for the next beta release (usually released on a Tuesday, so July 23rd). Apologies for the inconvenience and hopefully with a little patience we'll have this sorted soon.