Transition from mailgun org to gubernator-io org

[thrawn01]

Purpose

This is to transition the repo from github.com/mailgun/gubernator to github.com/gubernator-io/gubernator such that adding and removing contributors is simplified and does not require maintainers have access to the mailgun org.

Implementation

• Transition public container from ghcr.io/mailgun/gubernator to ghcr.io/gubernator-io/gubernator
• Rename all import paths from github.com/mailgun/gubernator to github.com/gubernator-io/gubernator
  *Ensure links to all external and github resources point to https://raw.githubusercontent.com/gubernator-io/gubernator/master
• Added CONTRIBUTING.md and CODE_OF_CONDUCT.md

Manual Process

• Ensure existing tags remain
• Transition old container tags to new repo
• Ensure github actions work as expected
• Create a new release in github.com/gubernator-io/gubernator which refers to the old repo release log
• Validate docker containers for the new release are built and can be accessed via ghcr.io/gubernator-io/gubernator
• Re-enable check-version.sh script in github actions after new release.

[thrawn01]

I'm holding this PR until we merge and update the other pending PR's on the old repo.

[Baliedge]

Recent versions of golangci-lint complain about this and prefer the explicit import name.