Updating a LinkSys WRT19000ACS from 18.06 to 22.03

I get this: The uploaded image file does not contain a supported format. Make sure that you choose the generic image format for your platform.

It's running: Powered by LuCI openwrt-18.06 branch (git-18.228.31946-f64b152) / OpenWrt 18.06.1 r7258-5eb055306f

I downloaded this: https://downloads.openwrt.org/releases/22.03.2/targets/mvebu/cortexa9/openwrt-22.03.2-mvebu-cortexa9-linksys_wrt1900ac-v1-squashfs-sysupgrade.bin

Please can you tell me which image I need and the link to obtain it?

You should probably force it, don't however go to 22.03, but 23.05 RC, pretty sure all the mvebu 22.03s have seriously crippled wifi.

Take a backup first. Do NOT save config over reflash. You will need to recreate your network config from scratch because the platform has moved to DSA.
See these relevant wiki links