fix: SQLSRV driver's decrement() method

[patel-vansh]

Description
As discovered by @paulbalandan in this comment, SQLSRV Database Driver was incorrectly adding instead of subtracting the decrement value when $castTextToInt was false.

This PR fixes that bug by flipping the sign in decrement() function.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[patel-vansh]

I am quite unsure about the tests in this case. Do I need to add a new file in tests/system/Database/Live/SQLSRV directory? Or add a particular test inside the normal IncrementTest.php file?

[michalsn]

The new class under the SQLSRV directory would be better. Please add tests for both increment() and decrement() with castTextToInt = false. You can use the existing job table and its integer created_at field for this.