fix #12785

• Only for new installations we use matomo.js|matomo.php through the API as default. For older installations we use piwik.js|piwik.php. This way we won't break anything when tracking code is embedded automatically (eg the case on 70k+ wordpress installations).
• The tracking code generator and "site without data" screens show the matomo endpoint though.
• Custom Piwik JS plugin has now an alias command ./console custom-matomo-js:update(won't update the plugin as it may break things, is complicated, etc. lets do this in Matomo 4.0 Rename CustomPiwikJs plugin to CustomMatomoJs (or CustomTrackerJs) #13604)
• The Custom JS plugin now updates /matomo.js and /piwik.js when generating the tracker file
• The Custom JS plugin diagnostic check may check whether matomo.js and piwik.js is writable depending on when Matomo was installed
• The Matomo endpoint can be enforced in the tracking API through a parameter (not really worth mentioning in the dev changelog as it is pretty much only for internal purposes).
• Plugins that hook to Piwik.getJavascriptCode could now in theory rename the tracker endpoints matomo.js|php to anything else but the plugin would need to ensure those files exist etc.

We now also record the version number that was used to install Matomo so we can easier make changes in the future without breaking things for existing users.