Unify MOODLE_310_STABLE and master branch into single MOODLE_34_STABLE branch

[cameron1729]

The master and 310 branches are almost at parity:

git diff --numstat master..MOODLE_310_STABLE
3       3       README.md
2       16      lib.php
1       1       tests/phpunit/robot_crawler_test.php

All of these are minor differences that can be reconciled. After retiring the MOODLE_310_STABLE branch we should create a MOODLE_34_STABLE branch with $version->supports = [34, 401]. As of creating this isssue, all actions in the reusable workflow pass on Moodle 3.4 up to Moodle 4.0 (assuming PHP version 7.1+) - so we can simply maintain one branch.