Update php5 and php7.0 references to php7.3

[adrianyorke]

Fixes: #1811

[adrianyorke]

Please note: the "unix:/var/run/..." syntax seems to have changed to "unix:/run/" in the php7.3 default config file. The changes in this PR use the revised syntax.

[aallan]

I'd encourage you to wrap this PR up in the next week or so as we're in the process of transitioning the documentation from the current Markdown-based source format to Asciidoc. At some point soon — probably around the end of June, beginning of July — we will freeze the current documentation repo. After that time contributions and PRs based on the Markdown source will not be accepted, and any PRs that are still open will be closed.

See #1911 for full details.

[adrianyorke]

@aallan This branch has no conflicts and is a relatively minor update. I have no further commits for this PR unless there are review observations - I don't see any? I recommend this is merged prior to asciidoc migration the transition process starts with an accurate base. Thanks for the reminder and best wishes with the transition.