WT-13764 Bump WiredTiger version to 12.0.0

[lukech]

Changes made in this PR:

• Bump the WiredTiger version to 12.0.0 right after cutting the 11.3.1 release.
• Add coverage for version 12.0 in Python compatibility tests.

[github-actions]

Thanks for creating a pull request! Please answer the questions below by editing this comment.

Type of change made in this PR

• Functional change
• Test-only change
• Refactor-only change
• Other non-functional change

What makes this change safe?

Answering this question helps the reviewers understand where they should focus their attention. Please consider these prompts:

• How risky is this change? Why?
  Low-risk as it would affect only the open-source release and corresponding documentation.
• What tests are you adding, changing or relying on? Why?
  Python compatibility tests - they are part of PR testing.
• What, if anything, are you concerned about that you'd like the reviewer to focus on?
  Nothing special.

References:

• Risk level guide
• Testing frameworks

Checklist before requesting a review

• I have performed a self-review of my code.
• I have made corresponding changes to the documentation, or no documentation change is needed.
• I have tests that show my change is correct, or have described above why functional test changes are not required.