Update the release scripts in the hack folder

[ciarams87]

Proposed changes

This commit updates the scripts we use to prepare the release commits.

• Because we no longer update the files separately in the master branch (we merge the release branch into master), we no longer need a separate file to prepare that commit.
• The files can be updated for a major release using hack/prepare-major-release.sh prev_ic_version ic-version prev_helm_chart_version helm-chart-version. The docs/content/releases.md file will still need to be updated manually.
• The files for a minor release can be updated as before using hack/prepare-minor-release.sh ic-version helm-chart-version. The changelog template has been updated for the new docs format.

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto master
• I will ensure my PR is targeting the master branch and pulling from my branch from my own fork