Update PHPStan & Psalm to the latest versions

[spaze]

Also:

• Added the final keyword to classes
• Added Override attribute to tests
• Simplified the timestamp calculation instead of typecasting the return values from microtime() or the $keyRegeneration property

[antonioribeiro]

Hey @spaze, sorry but making the main Google2FA class final is too heavy-handed. I don't want to make it unextendable. I'm pretty much with the "old" Taylor here:

"...in open source code it's so bad to come across a final class and you just can't extend it and, ugh, whatever."

[spaze]

You could still use composition instead of inheritance but we could debate it for a long time, especially when it comes to security-related classes. Psalm added the rule some time ago, but it could be ignored. Happy to do it and have at least the other commits in this PR, feel free to reopen :-)