Raise PHPStan level for the `amp/common` package to 5

[schlessera]

Feature description

The Common package (ampproject/common) is currently sitting at and enforcing level 4 of PHPStan static analysis.

There are 2 errors that need to be fixed so that we can raise the level to 5 and enforce this new quality level in Travis CI.

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Acceptance criteria

• PHPStan does not show any detected errors when run on level 5.

Implementation brief

• Change the level to 5 in the file phpstan.neon.dist.
• Run composer analyze.
• Fix detected errors until no more are found.

QA testing instructions

Demo

Changelog entry