feat: convert CEUs to decimal

[asadali145]

What are the relevant tickets?

Closes https://github.com/mitodl/hq/issues/5834

Description (What does it do?)

Converts CEUs from string to decimal to avoid issues.

How can this be tested?

• Checkout master branch and create the data
• Create a few certificate pages for program and course pages. Add CEUs, Keep a few CEUs empty, and override a couple.
• Create multiple course runs and program certificates.
• Now checkout this branch and apply the migration
• Test the following:
  • CEUs are visible on the product page if the certificate.CEUs exist, but are not visible if only override exists without the certificate.CEUs
  • CEUs are visible in the certificate preview if the certificate.CEUs, not visible if only override
  • CEUs are visible in the certificates if CEUs exist or Override is visible when CEUs exist.
  • Credits are correct in api/courses, should be null or string
  • Credits are correct in api/programs, should be null or string for each course and null or string for the program itself
  • CEUs are visible in the dashboard receipt.
  • I am not sure about the usage of build_course_run_credential and build_program_credential, But we should verify that CEUs are returned in the data. I verified it through shell
• Verify that nothing changes display-wise and no breaking changes.