feat: increase entropy of OTPs generated by code strategy

[aeneasr]

Moving forward, OTPs generated by the code strategy will match pattern [0-9a-zA-Z]{8} instead of [0-9]{6}. This increases entropy and makes it easier to defend against reverse brute force attacks.

See https://github.com/ory-corp/cloud/issues/3724

Related issue(s)

Checklist

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Further Comments

[aeneasr]

A test is needed to ensure the legacy code generation still works.

[tricky42]

Can we follow up on this to close: https://github.com/ory-corp/cloud/issues/3724?

[aeneasr]

Yes, just need to figure out how to change it without breaking customers' existing implementations - especially custom UIs.