Bump core dependencies minimum version to PHP 7.0

Can we please at least bump to 7.1?

Symfony 4.0 requires PHP 7.1.3

https://symfony.com/doc/current/reference/requirements.html

I think we have an existing issue that outlined how exactly we will increase the PHP version requirement? And afaik, updating to require PHP 7.1 will follow in a second step, AFAIK we won't be able to require symfony 4.0 in Drupal 8 anyway.

Is the issue title of this issue still accurate? What's achieved by PHP 7.0.8? I don't see where that comes into play in the patch.

Per #3020301-3: Remove composer integration PHP version test (rely on DrupalCI PHP version runs instead), because of Ubuntu, I think bumping to 7.0.8 is less problematic than bumping to 7.1.3, but I'm not clear on what it accomplishes, if it's insufficient to get to Symfony 4.

Sorry, I skimmed this issue too quickly. I see that the patch does update several libraries, like doctrine/inflector and phpunit to versions that require PHP 7.0 but not 7.1. Yes, I think that's a good step.

Tagging for release manager review to decide on when to commit such a change to HEAD. Both in terms of the PHP version bump and in terms of updating the MAJOR version of some of the libraries, such as PHPUnit.

In fact, I think it might be good to split this into two steps if possible: first update the libraries that only require a minor version update. Then do the libraries that require a major version update in a separate issue. Or are there cross-dependencies that make that impossible?

The official plan for 5.5 and 5.6 EOL is in this issue: #2842431: [policy] Remove PHP 5.5, 5.6 support in Drupal 8.7

Under that plan, we do not introduce any hard breaks to PHP 5 support in 8.7. It's a warning on update rather than an error if you use an old PHP version, because we'd rather you at least install your Drupal security updates. It's the official EOL, but for testing continuity and the continuous upgrade path, the hard breaks begin in 8.8 and we monitor the PHP 5 support with a nightly test. So this patch will need to be targeted for 8.8 (which will open in a few months) rather than 8.7. (It's definitely good to do the work now, though, to see the consequences.)

Regarding #6 and requiring PHP 7.1.3. We declared an EOL for Drupal's PHP 5 support a year ago, and at the time we made a decision to postpone a decision about PHP 7.0 support until we had more information. We also announced that when we did make a decision about 7.0, we would also announce that a year ahead of time.

Furthermore, recent d.o data makes it look like a lotof people are using 7.0, so it'd be pretty impactful for the ecosystem. For that reason, I think we need to revisit a 7.1 dependency later because we're not ready for that as a community now.

Also see #3020301: Remove composer integration PHP version test (rely on DrupalCI PHP version runs instead).