[ticket/12920] Add composer.json for phpbb/phpbb-core library

PHPBB3-12920
[ticket/12920] Add require for minimum php version

PHPBB3-12920
[ticket/12920] Turn phpBB composer.json into publishable full version

PHPBB3-12920
[ticket/12920] Output warning message about vendor cleaning target

PHPBB3-12920
[ticket/12920] Remove incorrect @ from self.version reference in composer.json

PHPBB3-12920
[ticket/12920] Fix sh syntax error in post-update-cmd script

PHPBB3-12920
[ticket/12920] Update composer.lock file after update nothing

PHPBB3-12920
[ticket/12920] Add missing comma

PHPBB3-12920
Merge pull request #2813 from naderman/ticket/12920

[ticket/12920] Add composer.json to phpBB/phpbb and fix the one in phpBB/

* naderman/ticket/12920:
  [ticket/12920] Add missing comma
  [ticket/12920] Update composer.lock file after update nothing
  [ticket/12920] Fix sh syntax error in post-update-cmd script
  [ticket/12920] Remove incorrect @ from self.version reference in composer.json
  [ticket/12920] Output warning message about vendor cleaning target
  [ticket/12920] Turn phpBB composer.json into publishable full version
  [ticket/12920] Add require for minimum php version
  [ticket/12920] Add composer.json for phpbb/phpbb-core library