Dropping PHP 7.x support

[reedy]

https://packagist.org/packages/jakobo/hotp-php/php-stats seems to suggest 7.3 has dropped off for a while, but maybe not worth that bump yet to just 7.4...

But PHP 8 eventually. Not sure if there's much functionality we'd gain/code we'd want to change

[thecodedrift]

I don't think there's much to change since the goal is stability. What's a reasonable cutoff in your opinion? For frontend code, we usually use 2% as the cutoff, but I'm concerned we'll be stuck with 7.4 forever if we use that as a guide.