fix: support TS 4.9 and drop support TS 4.5

[WikiRik]

Pull Request Checklist

• Have you added new tests to prevent regressions?
• If a documentation update is necessary, have you opened a PR to the documentation repository?
• Did you update the typescript typings accordingly (if applicable)?
• Does the description below contain a link to an existing issue (Closes #[issue]) or a description of the issue you are solving?
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Description Of Change

Now TS 4.9 is out we need to add that to our CI. We also drop support for TS 4.5 per our version policy since it has been out for a year.

[ephys]

If tests are green, let's go

Do you plan on adding 4.9 to our v6 CI?

[WikiRik]

Do you plan on adding 4.9 to our v6 CI?

Yes, in #15059 probably. But I need to figure out what to do with the Node 10 tests there, maybe I just need to undo some TS migration things