Add PHP 8.5 to the build matrix

[DannyvdSluijs]

Add PHP 8.5 to the build matrix. Since this version is not released it should allow failure. I've used this in the JsonMapper project as well.

https://github.com/JsonMapper/JsonMapper/blob/develop/.github/workflows/build.yaml#L84
https://github.com/JsonMapper/JsonMapper/blob/develop/.github/workflows/build.yaml#L111

[DannyvdSluijs]

Adding PHP 8.5 in the CI workflow doesn't work yet as Prophecy isn't supporting PHP 8.5 yet.
#815 prepares the workflow to add this once supported or when PHP 8.5 is GA (whichever comes first)

[stof]

Adding a CI job for an unreleased version of PHP is definitely the perfect use case for using --ignore-platform-req=php+ in Composer, to ignore the upper bound of the PHP requirement.