Drop PHP 7.4 support (#1244)

* drop php 7.4 support
update composer.json minimums, and run rector over the code to update what it can. changed some static analysis
config to fix some rector-induced failures.

* adding union typehints

* reformat constructor property promotions

* updating/removing phpdoc

* suppress protobuf extension complaint
submitted a PR upstream to fix phan stubs

* don't validate packages against 7.4

* remove php8 polyfill

* fix readonly comments

* apply trailing commas to multiline fixer
upgrade php-cs-fixer, and apply a new rule for multiline comments

* remove unreachable default match arms for protobuf serializer

* adding more union types to vars

* use weakmap directly

* remove handling for not-supported WeakMap

* spacing

Author: brettmc

JaegerBaggagePropagator.php

@@ -73,7 +73,7 @@ public function extract($carrier, PropagationGetterInterface $getter = null, Con
         $baggageBuilder = Baggage::getBuilder();
 
         foreach ($baggageKeys as $key) {
-            if (strpos($key, self::UBER_BAGGAGE_HEADER_PREFIX) === 0) {
+            if (str_starts_with($key, self::UBER_BAGGAGE_HEADER_PREFIX)) {
                 $baggageKey = substr($key, strlen(self::UBER_BAGGAGE_HEADER_PREFIX));
                 $value = $getter->get($carrier, $key) ?? '';
                 $baggageBuilder->set($baggageKey, rawurldecode($value));