Concurrency Tests - Update ExternalBatch for symfony/proces v4.1+

Concurrency Tests - Update ExternalBatch for symfony/proces v4.1+

Before
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`api_v3_JobProcessMailingTest::testConcurrency()` and `Civi\FlexMailer\ConcurrencyTest`
fail when executed on Drupal 10.

After
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Pass when executed on Drupal 10

Technical Details
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`symfony/process` v4.1 deprecated the usage of `new Process(string)` and
added `Process::fromShellCommandline(string)`. The `new Process(string)` was
dropped from v5.x and v6.x. So you get an unconditional failure on D10. (On
other platforms, it can also fail... depending on how you deal with
deprecations.)

In `civicrm-core:composer.json`, the minimum mversion of `symfony/process` is `4.4`.
So that means we can use the newer `Process::fromShellCommandline(string)`.