Update beta version format when running the prep-release script

[edif2008]

Overview

For Python, a beta version has the format x.y.zbw (e.g. 0.4.0b2).
This PR updates the prep-release.sh script to use this format.

How to test

1. Update the src/release/scripts/prep-release.sh and comment the following lines (L111-113):

   git add .
   git commit -S -m "Release v${version}"
   git push --set-upstream origin "${branch}"

2. Temporarily commit the change.
3. Run make prep-release
4. Try to provide a wrong beta version e.g. 0.5.0-beta.1
   • It should reject it and ask for the version again.
5. Provide the appropriate beta version e.g. 0.5.0b1
   • It should accept it and continue the process.
6. Continue the process (you can ignore editing the release notes)
7. Verify the version stored in version.py
   • It should contain the beta version in the appropriate format.
8. Reset your branch and remove the temporarily committed changes:

   git reset --hard HEAD~1