Comment # 16 on bug 1213438 from Takashi Iwai

You need to pass --oldpackage to zypper install commands.  And install all
three at once.  That is,
  zypper install --oldpackage
kernel-default-5.14.21-150500.1.1.g8d27c97.x86_64.rpm
kernel-default-extra-5.14.21-150500.1.1.g8d27c97.x86_64.rpm
kernel-default-optional-5.14.21-150500.1.1.g8d27c97.x86_64.rpm 

For choosing a boot kernel at GRUB, choose "Advanced options..." by the cursor
key, press RETURN.  Then you can choose a kernel to boot.  Select the right one
(it may appear at the bottom) *WITHOUT* recovery mode, and press RETURN.