You may want to clear your cache if an unknown number of the packages are corrupted.
sudo pacman -Sc
You could alternatively target the few with issues, ex:
paccache -rvk0 archlinux-keyring manjaro-keyring
Or
sudo rm /var/cache/pacman/pkg/*keyring*
Now for the download and apply bits, done semi-manually;
Get packages:
curl -O https://mirror.easyname.at/manjaro/pool/sync/archlinux-keyring-20240709-1-any.pkg.tar.zst
curl -O https://mirror.easyname.at/manjaro/pool/overlay/manjaro-keyring-20230719-3-any.pkg.tar.zst
Remove old keys:
sudo rm -r /etc/pacman.d/gnupg
Init keyring:
sudo pacman-key --init
Install the packages we downloaded:
sudo pacman -U manjaro-keyring*.pkg.tar.zst archlinux-keyring*.pkg.tar.zst
Populate keys:
sudo pacman-key --populate manjaro archlinux
And then of course see how are the things. We can include a mirror sort for extra giggles:
sudo pacman-mirrors --continent
sudo pacman -Syu