chore: Bump v4.20.0

[jstoobysmith]

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[github-actions]

Thank you for your PR! The following checklist can be used to ensure that the PR is as helpful as possible and to help maintain the quality of the project.

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Documentation

• I have updated the module doc-strings of any modules modified, or this comment is not relevant.
• I created a new module doc-string for any new modules created, including an overall description of the module and the main results, or this comment is not relevant.
• I have changed all doc-string occurances of decleration names which I have changed, or this comment is not relevant.
• For definitions and key theorems I have added, I have added an english-readable statement of the result, which is as self-contained as possible, and where appropriate contain a readable UTF-8 version of the result.
• Where appropriate and helpful I have included references to the original source of results in the module doc-string.

Lemmas and theorems

• I have ensured that the theorem key word is reserved for important theorems, and lemma is used for everything else.
• I have ensured that no lemmas or definitions are duplications of other results.

Layout

• I have ensured that results are in the appropriate modules.

Usage of AI

Please tick one that applies to this PR:

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• I have used basic AI coding assistance (e.g. Copilot) to generate repeative bits of code, but the lemmas and definitions are my own.
• I used AI to generate the structure of the code and/or guide its organization, but I have checked it for corectness.
• Other (please specify with a comment).