Replaced MySQL 5.7 installed from a PPA with an official Docker image

See https://travis-ci.org/doctrine/dbal/jobs/495144274 for the PPA issue.

CI: Test against PHP 7.4snapshot instead of nightly (8.0)

Fixed AppVeyor build configuration and the issue on SQL Server

PowerShell doesn't automatically handle exit codes of the invoked commands automatically, and $ErrorActionPreference = "Stop" doesn't seem working. Additionally, the ocular command is moved out of the "test_script" section to "after_test" since it's not part of the actual test.

PHPStan Level 7

Upgraded PHPUnit to 8.0

Replaced custom docker image for PostgreSQL with the official one

See https://travis-ci.org/doctrine/dbal/jobs/488051004 for the dependency issue.

PHPStan Level 6

PHPStan Level 5

PHPStan Level 4

Updated PHPStan to v0.11

Index length can be a `string`: ensure that it is an integer when read by the `MySqlSchemaManager`

Add column charset for MySQL

Consider column lengths when comparing indices

`CREATE_OPTIONS` is nullable in MySQL

Introduced a smoke testing phase on Travis to run SQLite tests and CS checks first

CI: Test against MySQL 8.0 on Travis

Removed unused build files

Travis CI: PHP 7.3 tests on MySQL 8.0

Link to Slack instead of Gitter

Gitter has been deprecated in favor of Slack for all Doctrine purposes.