udpate cspo builder image

[kranurag7]

cluster-stack-provider-openstack/images/builder/Dockerfile

Line 51 in b94dfb6

FROM docker.io/library/golang:1.21.6-bullseye@sha256:a8712f27d9ac742e7bded8f81f7547c5635e855e8b80302e8fc0ce424f559295

this needs to be bumped to 1.21.8 for this one to pass.

See the following message:

go: module github.com/gophercloud/utils/v2@v2.0.0-20240228132853-8515e998ecab requires go >= 1.21.6; switching to go1.21.8

I'll update the builder image.

Originally posted by @kranurag7 in #112 (comment)