Update OPA images to use non-root uid/gid by default

Currently all OPA image variants except "rootless" use
uid/gid 0 (i.e. root). Per container security best
practices it is better to run as non-root. So now OPA
defaults to non-root uid/gid in images. If root user
if needed, it can be explicitly set.

The "rootless" image variant is no longer needed and
will be not published in future releases.

Also currently the debug variant is published
for `linux/amd64` platform. For `linux/arm64` only static
images are generated. The debug variant can be useful
for debugging purposes and hence this change adds that
to the static image which can then be used on amd64 and arm64 arch.

Signed-off-by: Ashutosh Narkar <anarkar4387@gmail.com>