Raise minimum supported PHP version to 7.3

[albe]

Since PHP 7.2 is EOL in November (https://www.php.net/supported-versions.php), we should phase out support for this version.

• Remove 7.2 builds from travis (TASK: Remove PHP 7.2 from build matrix #2235)
• Raise required PHP version in composer.json (!!! TASK: Raise minimum PHP version to 7.3 #2237)
• Adjust dependencies (!!! TASK: Update Doctrine Migrations to 3.0 #1880)
• Adjust Neos (TASK: Remove PHP 7.2 builds neos-development-collection#3171 & !!! TASK: Raise minimum PHP version to 7.3 neos-development-collection#3172)
• Update https://docs.neos.io/cms/installation-development-setup/system-requirements#php-version-matrix

We should also go forward with adding support for PHP 8 (see #2233)

[bwaidelich]

The docs still need to be adjusted: #2281