Add PHP8.1 testing to Travis environment

[asmecher]

OJS/OMP/OPS 3.3.0 and main (pre-3.4) should be tested against PHP8.1.

PRs:

• pkp/pkp-lib#7690 Add PHP8.1 testing to stable-3_3_0 ojs#3296

[asmecher]

It looks like PHP8.1 support is not yet great on Travis.

• For 20.04, it's falteringly available: https://travis-ci.community/t/php-8-1-support/12439/23
• For older distros, may not be available at all yet (including the one we use for stable-3_3_0).

There are a lot of discussions about backwards compatibility being broken in the PHP internals list:
https://externals.io/message/115683

This is also relevant: https://wiki.php.net/rfc/deprecate_null_to_scalar_internal_arg

For some reason I haven't been able to track down yet, PHP really wants to dump these to stdout, breaking AJAX interactions and redirects. This seems to be due to something in OJS or a dependency; a straight-up call to an affected function in vanilla PHP doesn't seem to mind. (Looking at you, Smarty?)

---

Edit: It looks like 8.1.0 is available specifically for most distros, but there is no 8.1 alias as there is for previous major branches.

Smarty is indeed pickier, and many of the adjustments have gone there. In particular, passing a null through |escape now causes an error due to https://wiki.php.net/rfc/deprecate_null_to_scalar_internal_arg.

[asmecher]

Until we're early in a 3.3.0-x release cycle, and until 3.1.0 gets a little stabler, I've committed the changes to main and not stable-3_3_0.