drop php 8.0 support (#1256)

* bump php to 8.1, apply rector
* readonly and typehints
* suppress a dodgy readonly property conversion
* remove some redundant readonly phpdocs
* dont run workflows against 8.0
* more readonly and typehints
* phpdoc reformat
* typehints
* remove php81 polyfill
* revert a rector BC breaking change, and disable that rector for ParentBased

Author: brettmc

composer.json

@@ -17,7 +17,7 @@
         }
     ],
     "require": {
-        "php": "^7.4 || ^8.0",
+        "php": "^8.1",
         "open-telemetry/api": "^1.0",
         "open-telemetry/context": "^1.0"
     },