Add Pg18+ support

[AntonOfTheWoods]

Draft PR exploring support for postgres 18+.

This should be strictly identical for mysql/sqlite.

There is a "nasty hack" that simply discards db_index parameters passed to case insensitive columns for postgres to maintain compatibility. It appears that significant code changes would be required for selecting field types based on if connection.vendor == "postgres". Given postgres' closest option that allows for likes (>=pg14, rather than the current types which require >=pg18) and indexes is the extension citext, and citext has been deprecated in postgres and the specialised Django class (CIText) has been removed, it seems like immediately introducing technical debt.

For the two columns I could find in openedx-learning or edx-platform that had case-insensitive indexes - oel_collections.Collection and oel_tagging.Taxonomy - the first is just deleted straight after in a subsequent migration and I couldn't find anywhere where the index might actually add any value for the second. My confidence level is very low on that though.

I couldn't find any mentions of MultiCollationCharField/MultiCollationTextField outside of these two projects on all of public Github. Obviously that doesn't mean people aren't using it but it's promising.

Would fix #422

[openedx-webhooks]

Thanks for the pull request, @AntonOfTheWoods!

This repository is currently maintained by @axim-engineering.

Once you've gone through the following steps feel free to tag them in a comment and let them know that your changes are ready for engineering review.

🔘 Get product approval

If you haven't already, check this list to see if your contribution needs to go through the product review process.

• If it does, you'll need to submit a product proposal for your contribution, and have it reviewed by the Product Working Group.
  • This process (including the steps you'll need to take) is documented here.
• If it doesn't, simply proceed with the next step.

🔘 Provide context

To help your reviewers and other members of the community understand the purpose and larger context of your changes, feel free to add as much of the following information to the PR description as you can:

• Dependencies

  This PR must be merged before / after / at the same time as ...

• Blockers

  This PR is waiting for OEP-1234 to be accepted.

• Timeline information

  This PR must be merged by XX date because ...

• Partner information

  This is for a course on edx.org.

• Supporting documentation
• Relevant Open edX discussion forum threads

🔘 Get a green build

If one or more checks are failing, continue working on your changes until this is no longer the case and your build turns green.

🔘 Update the status of your PR

Your PR is currently marked as a draft. After completing the steps above, update its status by clicking "Ready for Review", or removing "WIP" from the title, as appropriate.

---

Where can I find more information?

If you'd like to get more details on all aspects of the review process for open source pull requests (OSPRs), check out the following resources:

• Overview of Review Process for Community Contributions
• Pull Request Status Guide
• Making changes to your pull request

When can I expect my changes to be merged?

Our goal is to get community contributions seen and reviewed as efficiently as possible.

However, the amount of time that it takes to review and merge a PR can vary significantly based on factors such as:

• The size and impact of the changes that it introduces
• The need for product review
• Maintenance status of the parent repository

💡 As a result it may take up to several weeks or months to complete a review and merge your PR.