Support PHP 8.5

[Jan-Schuppik]

Changes made:

PHP 8.4:

• Function parameters that are null by default must be declared nullable.

PHP 8.5:

• Using null as an array offset is deprecated.
• Remove deprecated method ReflectionProperty::setAccesible()

Other changes:

• Update composer package guzzlehttp/psr7

resolves #181

[sukhwinder33445]

Looks good

[lippserd]

In addition to the requested changes, I have a few comments on the commit messages and descriptions:

For 982ef28, please use something like the following, as suggested in Icinga/ipl-stdlib#61 (review):

PHP 8.4: Change implicit nullable type declaration to explicit

Since PHP 8.4 implicitly nullable parameter types are deprecated.

The reason for upgrading guzzlehttp/psr7 in d5b0989 is misleading, as version 2.8.0 introduces PHP 8.5 support and is a dependency for this library. In general, our ipl-* repositories build our Icinga PHP library, not the other way around. Therefore, we should never have such "streamline" commits. Something similar was also discussed/addressed in Icinga/ipl-validator#38.

For 41413f0, the commit message could be worded more precisely, and a description justifying the removal should be added, e.g.:

PHP 8.5: Remove deprecated method `ReflectionProperty::setAccessible()`

Since we require PHP >= 8.2 and since calling this method has no effect as of
PHP 8.1.0, as all properties are accessible by default, it is safe to simply
remove this method call.

Finally, 3ef2e00 could be rephrased to PHP 8.5: Replace usage of `null` as array offset. A description that only contains a reference to the PHP documentation is not necessary, but something like the following:

Since PHP 8.5 using `null` as an array offset is deprecated. Code relying on 
`null` being silently converted to an empty string must be replaced and
discouraged in PHPDocs.

[lippserd]

And please also add a PR description.

[sukhwinder33445]

I have adjusted the classes SelectElement, RadioElement and the tests. Please have a look @lippserd.

[lippserd]

Both concerns regarding the radio element also apply to the select element.

[lippserd]

Since explicit is better than implicit, I also suggest adding new assertNull() calls after the setValue() calls at the end of testNullAndTheEmptyStringValuesAreEquallyHandled() for both Radio and SelectElementTest. If I'm not mistaken, there is no check to validate if setValue(''|null) leads to getValue(): null.

[lippserd]

Thanks 👍