[CodeQuality] Use atLeast() instead of exactly() in DecorateWillReturnMapWithExpectsMockRector

[TomasVotruba]

willReturnMap() only maps argument combinations to return values — it does not enforce a strict call count. Decorating with expects($this->exactly(N)) forces the mock to be called exactly N times (N = map entry count), which produces false test failures when the mocked method is called fewer or more times than there are map entries.

Switching to atLeast(N) keeps a lower-bound expectation without over-constraining the call count.

Before

$someMock->method('some')->willReturnMap([1, 2]);

After

-$someMock->method('some')->willReturnMap([1, 2]);
+$someMock->expects($this->atLeast(2))->method('some')->willReturnMap([1, 2]);