Deprecate XP annotation syntax

[thekid]

The following code (which creates a parse error in PHP 8!) will throw deprecation warnings with PHP 7 when accessing annotations reflectively.

class DemoTest {

  #[@test]
  public function can_create_demo() {
    // ...
  }
}

Causes E_USER_DEPRECATED: "XP annotation syntax is deprecated in DemoTest, line 6"

Part 2 of xp-framework/rfc#336

[thekid]

Tests are currently failing for some weird and unreproducible reason, see #285

Temporary issue with Github actions, fixed now.