feat: remove 2048-bit restriction for yubikey sizes

[ronantakizawa]

Description (e.g. "Related to ...", etc.)

Remove 2048-bit restriction for key sizes. See #446

Code review checklist (for code reviewer to complete)

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• Title summarizes what is changing
• Commit messages are meaningful (see this for details)
• Tests have been included and/or updated, as appropriate
• Docstrings have been included and/or updated, as appropriate
• Changelog has been updated, as needed (see CHANGELOG.md)

[ronantakizawa]

Users can set their own key size via CLI (taf yubikey setup-signing-key --key-size 2048), but it currently only supports 1024 and 2048-bit keys.

Based on this article:Yubico/yubico-piv-tool#58.

Repos can be signed by 4096-bit keys.

4096-bit keys are supported on OpenPGP mode but not on PIV (What we currently use).

[renatav]

Since this does not fully resolve the issue, change the PR description to see #issue num. We can merge this in though once it's ready. We'll see if we can replace PIV, but that does not have to be done now