Adding migration that will replace root_dir variable with project_dir

[escopecz]

Q	A
Bug fix? (use the a.b branch)	[y]
New feature/enhancement? (use the a.x branch)	[ ]
Deprecations?	[ ]
BC breaks? (use the c.x branch)	[ ]
Automated tests included?	[ ]
Related user documentation PR URL	mautic/mautic-documentation#...
Related developer documentation PR URL	mautic/developer-documentation#...
Issue(s) addressed	Fixes #12516

Description:

In #11145 we've changed all values of %root_dir% to %project_dir% but this still must be done in local.php during upgrade from M4 to M5 so users don't have to panic when the Mautic instance will be broken due to this issue.

Steps to test this PR:

1. Open this PR on Gitpod or pull down for testing locally (see docs on testing PRs here)
2. Add these lines to your local.php:

        'testing_path1' => '%kernel.root_dir%/../../var/tmp',
	'testing_path2' => 'some/%kernel.root_dir%/var/tmp',

These are the 2 options being replaced in #11145.
3. Run bin/console doctrine:migrations:migrate. You may have to hit the Enter if you are asked some question.
4. Once the migration has run, your local.php values should be changed to:

        'testing_path1' => '%kernel.project_dir%/../var/tmp',
	'testing_path2' => 'some/%kernel.project_dir%/app/var/tmp',

[codecov]

Codecov Report

Merging #12526 (de40f24) into 5.x (3a3698a) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                5.x   #12526      +/-   ##
============================================
- Coverage     57.28%   57.28%   -0.01%     
  Complexity    33535    33535              
============================================
  Files          2158     2158              
  Lines        101350   101350              
============================================
- Hits          58057    58056       -1     
- Misses        43293    43294       +1     

see 1 file with indirect coverage changes

[fedys]

Thanks @escopecz 👍