feat(oci): publish helm chart to xpkg.upbound.io

[haarchri]

Description of your changes

Publish Helm Chart in Addition to xpkg.upbound.io/upbound/universal-crossplane remove unused variable BOOTSTRAPPER_TAG

OCI Flow: During the publish target, the chart will be made available in xpkg.upbound.io/upbound/universal-crossplane

Examples:

branch: main

xpkg.upbound.io/upbound/universal-crossplane:1.17.0-up.1.rc.0.2.g095726e

branch: release-1.17

xpkg.upbound.io/upbound/universal-crossplane:1.17.0-up.1

we will remove the publish in charts.upbound.io in a later release!

Fixes #

I have:

• [x] Read and followed Upbound's contribution process.
• [x] Run make reviewable to ensure this PR is ready for review.
• [x] Added backport release-x.y labels to auto-backport this PR, as appropriate.

How has this code been tested

we use the same flow with mcp-connector