After T192166 is done... We can bump to newer versions of the symfony libraries (>4.0) that don't have HHVM support [1]. This is in MW core, extensions, vendor, and the composer docker image we use for CI
[1] https://symfony.com/blog/symfony-4-end-of-hhvm-support26570726f6475636520796f757220627567207573696e67206120726563656e742076657273696f6e206f662074686520736f6674776172652c20746f2068652077696b6920636f6e74656e74206c616e67756167652e0a0a5468616e6b20796f752e0a546167730a436865636b557365720ad70a436f6e6e65637465642d4f70656e2d48657269746167652d42617463682d75706c6f61647320285241c42d4b4d425f315f323031372d3032290ad70a54616d696c2d53697465730ad70a47616d6570726573730ad70a48617368746167730ad70a4a4144450ad70a4b6172746f456469746f720ad70a4c616e67756167652d323031382d4170722d4a756e650ad70a4e65772d456469746f722d457870657269656e6365730ad70a4d61696c0ad70a5443422d5465616d0ad70a53756273637269626572730a4465736372697074696f6e20507265766965770a436f6e74656e77a6f6e652073657474696e6720696e20796f75722070726f66696c652c20636c69636b20746f207265636f6e63696c652e