[FT] Drop PHP 7.2 and add PHP 7.4 and 8.0 (flix-tech#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 (flix-tech#51)

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

allow php8 (flix-tech#51)

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

Add support for psr cache interfaces (flix-tech#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!

Update `flix-tech/avro-php` dependency (flix-tech#36)

Co-authored-by: @nick-zh
Co-authored-by: @tPl0ch

[DOC] Preparing docs for new major release 7.0.0 (flix-tech#31)

Fixes flix-tech#26

* [DOC] Preparing docs for new major release 7.0.0
* [DOC] Fix type in `README.md`

[FT] Use new 3.0 version of `flix-tech/avro-php` (flix-tech#24)

[CI] Use wikimedias Avro library (flix-tech#22)

[FX] Fixing dealing with exceptions wrapped in fulfilled promises (fl…

…ix-tech#15)

[RELEASE][HOTFIX] Correct Stream processing with newer Guzzle versions (

flix-tech#14)

* [FX] Also add throwing values in rejection callbacks

* [FX] Also add throwing values in rejection callbacks for CachedRegistry

* [FX] Use correct stream string access in `ExceptionMap`