CI: Aggregate coverage from multiple jobs

Added badges for the develop branch in README

Removed hard-coded configuration filenames from the test runner

Add sidebar.rst and cleanup docs homepage.

Consistent PHPUnit configuration across environments

Add warning about MySQL JSON key sorting

See discussion at https://github.com/doctrine/doctrine2/issues/7187

Use newer PHPUnit to prevent crashes on failures

Cleanup docs folder after launch of new website.

1. XSD location pointing to the vendored file.
2. Removed `backupGlobals="false"`.
3. Added `failOnRisky="true" failOnWarning="true"`.
4. Added `beStrictAboutOutputDuringTests="true" beStrictAboutTodoAnnotatedTests="true"`.
5. Added code coverage whitelist.

Bump version to 2.8.x-dev

Fix compatibility for pre-2.7 DateIntervalType format

Fix #3082: Add BC for PDO-only fetch modes

CI: Add missing listener for MariaDB @ mysqli

Other drivers (including the cache) never supported fetch modes that are
not listed in `FetchMode`. Converting between the contants is only done
for PDO, so BC only needs to be restored here. PDO will validate the
fetch mode, we don’t have to.

Tests: remove implicit verbose flag

Document the correct way of configuring a MariaDB database with serverVersion

[bug] Don't skip column options.

Add symfony tests listener

It is required when using annotations to detect deprecation errors.

README: Add 2.7, drop 2.5

CS bump to 4.0, imports for global functions & constants

Add tests for column collation to prove it works