Pages supports points, centimeters, or inches from its Preferences : Ruler panel. Once you have set the preferred unit, select your image, and from the Image panel, choose Edit Mask and on it, select the crop tool:
Follow these instructions: Edit an image in Pages on Mac - Apple Support
Any reason why you have not updated Big Sur to 11.7.10 and obtained Pages v12.1 for it?
If you want Pages v12.1, you need to launch the Mac App Store and sign in with the same Apple ID as you used to previously install it. Press option+0 (zero) to open the Purchased panel and Pages will likely appear there with a blue cloud download icon. Click that and do not choose Cancel on any subsequent dialogs and you should have Pages v12.1, the last compatible version for Big Sur installed.