Update exclusion path for composer-classes.php

[boboldehampsink]

Description

Fixes

PHPStan process exited with error  filteredErr=Invalid entry in excludePaths:
Path "/vendor/craftcms/cms/src/config/composer-classes.php" is neither a directory, nor a file path, nor a fnmatch pattern.
If the excluded path can sometimes exist, append (?)
to its config entry to mark it as optional. Example:
parameters:
excludePaths:
analyseAndScan:
- vendor/craftcms/cms/src/config/composer-classes.php (?)  rawErr=Invalid entry in excludePaths:
Path "/vendor/craftcms/cms/src/config/composer-classes.php" is neither a directory, nor a file path, nor a fnmatch pattern.

If the excluded path can sometimes exist, append (?)
to its config entry to mark it as optional. Example:

parameters:
	excludePaths:
		analyseAndScan:
			- vendor/craftcms/cms/src/config/composer-classes.php (?)

Related issues

Fixes #2

[boboldehampsink]

Perhaps even better to just remove it, as #4 does