Automate Release Docs Release Tags

[danehans]

https://github.com/envoyproxy/gateway/pull/303 introduced a release-specific guide. The release references should be automatically updated when a release is cut.

• Update the make release-manifests Make target to release artifacts.
• Update release-manifests.sh to release-artifacts.sh
• Update the release-artifacts.sh to replace the previous release tag with the new release tag.

[Xunzhuo]

Though I have some questions for this, but you can assign this to me. I will take a look into this issue, to find out the best practice for this.

[Xunzhuo]

But I think this is in low priority, let us discuss this after releasing v0.2.0-rc1, no hurry for this:)

[danehans]

Note that the quickstart from the main branch should not use release tags. This could cause considerable changes to the steps of the quickstart guide.

[danehans]

Fixed by https://github.com/envoyproxy/gateway/pull/320