[DEPS] Also allow v2.0 of `guzzlehttp/promises` (#77)

Update flix-tech/avro-php (#75)

This upgrades to the latest `flix-tech/avro-php` version and adds PHP 8 CI checks.

Fix jsonSerialize on PHP 8.1+ (#71)

* Fix jsonSerialize on PHP 8.1+

```
During inheritance of JsonSerializable: Uncaught ErrorException: Return type of FlixTech\SchemaRegistryApi\Schema\AvroReference::jsonSerialize() should either be compatible with JsonSerializable::jsonSerialize(): mixed, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice
```

* Use ReturnTypeWillChange to be PHP 7 compatible

Fix XDebug setting again

Allow guzzlehttp/psr7 ^1.7|^2.1 (#66)

Add support for guzzle 7 (#64)

Once the UriTemplate class was heavily used and it had been removed from
Guzzle 7 it was not possible to add support for the next guzzle major
version.

Replace usages of UriTemplate with sprintf calls and update
composer.json with support for guzzle 7.

Fixes #57
Fixes #55

[FT] Drop PHP 7.2 and add PHP 7.4 and 8.0 (#53)

* Increase default versions for PHP and XDebug in `Dockerfile`
* Use PHP 7.4 in PHP shell wrapper
* Install XDebug from source in `Dockerfile`
* Use composer v2 in `Makefile`
* Remove now useless `--no-suggest` option
* Migrate to new PHPUnit config format
* Add `wait-for-it` scripts
* Add GitHub actions CI scripts
* Remove travis
* Update `Makefile`
* Remove custom XDebug options in `Dockerfile`
* Remove phpstan from `composer.json`
* Add composer binary to docker image in `Dockerfile`
* PHPUnit v 9.4 is required for PHP8 compat
* Add `phpstan.phar` to `.gitignore`
* Fix badges in `README.md`
* Fix copypasta from avro serde repository
* Require guzzle promises `^1.4.0` for PHP 8.0 support
* Add required env vars for integration test

allow php8 (#51)

* allow php8
* add nightly (8rc)
* adjust ci

allow php8 (#51)

* allow php8
* add nightly (8rc)
* adjust ci

Add support for psr cache interfaces (#37)

* Add support for psr-6 with the CacheItemPoolAdapter
* Add support for psr-16 with the SimpleCacheAdapter
* The package guzzlehttp/psr7 from version 1.7 breaks the tests, so 
  constraint it at version <1.7

Thanks to @fcoedno for the contribution!