Merge pull request #12 from academe/php8_1

PHP 8.1 minimum

Merge pull request #6 from Rockerby/patch-1

Add base64 encoding of 3DS data

Issue #4 switch to new Opayo URLs

Updated README references

Fix documentation reference

Add TOC

Update dependencies

Bring the package up-to-date.

* PHP 7.4+/8.0+ support only.
* Issue #57 to get AVS CVC check details.

Issue #58 Remove embarrassing var_dump()

Fixes to currency and amount (but don't use these anyway - use moneyp…

…hp/money).