Issue #1272 - SSP refuses to use temp dir if it doesn't own it, even though it can write to it (#1314)

* Issue #1272 - SSP refuses to use temp dir if it doesn't own it, even though it can write to it
This has been addressed using the is_writable function instead of checking for UID, which only works in a Linux environment

Co-authored-by: Aaron St. Clair <astclair@ecrs.com>

* Raise minimum PHP version to 7.0

* Remove tests pre-PHP 7.2

* Upgrade dev dependencies

* Ignore tests for deprecated class

* Add typehints; not touching public API

* Remove none-array replacements-param; old behaviour from pre-1.4 release

* Psalm fixes

* Add upgrade notes

* PSR-12

PSR-12 compliancy

Stream wrappers were assumed to be relative paths. When using a stream wrapper (e.g. s3://) it was appended to the given base path.

When opcache.validate_timestamps is disabled, then the new metadata will not be read after a metarefresh.
This can be solved by adding the metadata file to an opcache blacklist, but calling opcache_invalidate()
after writing a file is a nice out-of-the-box solution.

Hopefully, this will enable everybody that is using simplesamlphp to disable opcache.validate_timestamps
without running into problems.

To put it differently, to avoid OSX machines being identified as windows boxes, “darwin” should be evaluated *before* “win”.

Note that when normalising the directory separator, using
DIRECTORY_SEPARATOR causes tests to fail on non-Windows platforms
as its value is (correctly) "/".

Use InvalidArgumentException instead of SimpleSAML_Error_Exception when dealing with wrong input parameters.

SimpleSAML_Utilities::resolvePath was moved incorrectly to \SimpleSAML\Utils\HTTP instead of \SimpleSAML\Utils\System. Fix that.

Throw an exception on bad input parameters instead of using assert() in SimpleSAML_Utils_System::writeFile().

Schedule SimpleSAML_Utilities::writeFile() for removal.

Schedule SimpleSAML_Utilities::getTempDir() for removal.

Refactor SimpleSAML_Utilities::isWindowsOS() to SimpleSAML_Utilities::getOS(). Reimplemented to support most operating systems.

Schedule SimpleSAML_Utilities::isWindowsOS() for removal.