[Slim] Migration to PHP7

Created by: ybelenko

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.4.x, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

Migration to PHP7 requested in issue #1246 (closed)

I didn't add typehints to functions because we have expressive phpdoc comments.

Output from executed tests under openapi-generator/samples/server/petstore/php-slim:

$ composer test
> phpunit --testsuite Apis
PHPUnit 7.4.0 by Sebastian Bergmann and contributors.

..................................                                34 / 34 (100%)

Time: 498 ms, Memory: 4.00MB

OK (34 tests, 0 assertions)
> phpunit --testsuite Models
PHPUnit 7.4.0 by Sebastian Bergmann and contributors.

...............................................................  63 / 133 ( 47%)
............................................................... 126 / 133 ( 94%)
.......                                                         133 / 133 (100%)

Time: 2.44 seconds, Memory: 6.00MB

OK (133 tests, 0 assertions)

$ composer run phplint
> phplint ./ --exclude=vendor
phplint 1.0.2 by overtrue and contributors.

No config file loaded.

.......................................................................................

Time: 4 secs    Memory: 2.0 MiB Cache: No

OK! (Files: 87, Success: 87)

$ composer run phpcs
> phpcs ./ --ignore=vendor --warning-severity=0 --standard=PSR12

cc @jebentier @dkarlovi @mandrean @jfastnacht @ackintosh