Add return type annotation to future proof PSR-6 compatibility

[debo]

Question	Answer
Bug fix?	yes
New feature?	yes
BC breaks?	no
Deprecations?	no
Tests pass?	yes

Description

While working on my open source project I noticed that my build was breaking due to warning raised by my tests. I have a transient dependency to this package and it's raising multiple potential, future incompatibility warning in the form of:

1x: Method "Psr\Cache\CacheItemPoolInterface::methodName()" might add "type" as a native return type declaration in the future. Do the same in implementation "Cache\Adapter\Common\AbstractCachePool" now to avoid errors or add an explicit @return annotation to suppress this message.

I decided to add the annotation for now rather than change the implementation and leave that for a future date.

UPDATE: I think this changes is still a nice to have, but it also turns out that the warning was raised by symfony/phpunit-bridge. Credits to @mareg and @ciaranmcnulty for the additional input.

PS: It would be really nice if you could also add the hacktoberfest-accepted label to this PR. I hope it helps. Thank you.