Add support for psr/http-message ^2.0

[rotexdegba]

Changed psr/http-message requirement to ^1.1 || ^2.0 in composer.json.

Note that in dev environment (ie. cloning this repo with this pull request merged and running the tests in PHP 7.4) psr/http-message ^1.1 always gets pulled in by composer for PHP 7.4 users because PSR 7 implementations like laminas/laminas-diactoros ^2.17 in the require-dev section of https://github.com/slimphp/Slim/blob/4.x/composer.json requires psr/http-message ^1.0.

For users requiring slim/slim in their applications and using a PSR 7 implementation like nyholm/psr7 ^1.8 which requires psr/http-message ^1.1 || ^2.0 ( https://github.com/Nyholm/psr7/blob/1.8.1/composer.json ), psr/http-message ^2.0 will be pulled in for those applications.

Closes #3296

[coveralls]

coverage: 99.427%. remained the same
when pulling 187172d on rotexdegba:github-issue-3296-psr-http-message-version-requirement-bump-up
into 56ead41 on slimphp:4.x.

[kosciuk]

composer require slim/psr7

Problem 1
- slim/psr7[0.1.0, ..., 0.6] require php ^7.1 -> your php version (8.3.0) does not satisfy that requirement.
- slim/psr7[1.0.0, ..., 1.2.0] require php ^7.2 -> your php version (8.3.0) does not satisfy that requirement.
- slim/psr7[1.3.0, ..., 1.6.1] require psr/http-message ^1.0 -> found psr/http-message[1.0, 1.0.1, 1.1] but the package is fixed to 2.0 (lock file version) by a partial update and that version does not match. Make sure you list it as an argument for the update command.
- Root composer.json requires slim/psr7 * -> satisfiable by slim/psr7[0.1.0, ..., 0.6, 1.0.0, ..., 1.6.1].