Symfony version(s) affected: Many (under test suite for PHP 5.6-7.2)

Description
PHPUnit maintainers probably changed tags convention and all tags in X.Y format were changed to X.Y.0. Since symfony/phpunit-bridge defines default PHPUnit versions in old format, we get such errors in CI pipelines (below for bridge v3.2.0):

$ ./vendor/bin/simple-phpunit $PHPUNIT_COVERAGE_FLAGS

Warning: fopen(https://github.com/sebastianbergmann/phpunit/archive/5.3.zip): failed to open stream: HTTP request failed! HTTP/1.0 404 Not Found
 in /builds/<project>/vendor/symfony/phpunit-bridge/bin/simple-phpunit on line 41

Warning: stream_copy_to_stream() expects parameter 1 to be resource, boolean given in /builds/<project>/vendor/symfony/phpunit-bridge/bin/simple-phpunit on line 41

How to reproduce
Use bridge <4.1.8 which download ZIP package instead creating project through Composer.

Possible Solution
Change default versions to X.Y.Z format

Additional context
Feel free to close it if you don't want to support older versions of bridge, but I think it wouldn't be BC break to make such fixed for each branch <4.1.