Fix php8.1 deprecations, run tests on php8.1

[bastien-phi]

Another way to fix the deprecations in to add return types.

Let me know if you prefer this way

[mdumoulin]

Hi @msarca @sorinsarca and thanks @bastien-phi. The fix seems alright to me and similar to chosen solution in json-schema : opis/json-schema#96. Can we expect a merge soon ?

[sorinsarca]

Released in v2.0.1

[mdumoulin]

@sorinsarca Thanks !

[matiux]

Same error with opis/string 2.0.1

  1x: Method "ArrayAccess::offsetSet()" might add "void" as a native return type declaration in the future. Do the same in implementation "Opis\String\UnicodeString" now to avoid errors or add an explicit @return annotation to suppress this message.

  1x: Method "ArrayAccess::offsetUnset()" might add "void" as a native return type declaration in the future. Do the same in implementation "Opis\String\UnicodeString" now to avoid errors or add an explicit @return annotation to suppress this message.

[ricky-rockstar]

Same error with opis/string 2.0.1

  1x: Method "ArrayAccess::offsetSet()" might add "void" as a native return type declaration in the future. Do the same in implementation "Opis\String\UnicodeString" now to avoid errors or add an explicit @return annotation to suppress this message.

  1x: Method "ArrayAccess::offsetUnset()" might add "void" as a native return type declaration in the future. Do the same in implementation "Opis\String\UnicodeString" now to avoid errors or add an explicit @return annotation to suppress this message.

Is this going to be fixed?