scan code for legacy things

[dbu]

search for

• @deprecated

• PHP_VERSION_ID

• Kernel::MAJOR_VERSION

• class_alias

• method_exists

• scan the documentation for deprecated / legacy / versionadded and clean up

(not necessarily one huge pull request but smaller requests to fix things one-by-one)

[dbu]

part has been done in #617

[usu]

Also search for

• class_exists

[dbu]

fixed in #625 - the remaining class_exists are for optional dependencies.