Add milvus 3.0.0 to build matrix

[tamalsaha]

Adds 3.0.0 to the hand-maintained milvus build matrix (milvus is in the generator skip-list, so the workflow matrix is the source of truth — same as #66).

Requires appscode-images/milvus#2 to merge first: the 3.0 base image ships its own uid-999 user and the current Dockerfile's useradd fails on it. After merging both, dispatch build-milvus, then follow up with a library/milvus/promote_tags.txt PR (3.0.0_<date>) and dispatch promote (the #68/#54 flow) to publish ghcr.io/appscode-images/milvus:3.0.0.

Validated end to end with a locally built 3.0.0 image on a KubeDB k3s cluster (standalone + distributed provision, 2.6.11→3.0.0 UpdateVersion with data intact, full ops suite).