Add CA 2021 into CodeAnalysis config

[buyaa-n]

CA2021: Do not call Enumerable.Cast<T> or Enumerable.OfType<T> with incompatible types is a new analyzer that added in January.

Because it is Warns by default it caused build failure on automation PR that updates analyzer feed which caused by some edge case scenario bug.

Because the bug could cause a failure in other repos we lowered the severity of the analyzer until those bugs get fixed. Now the fix is ready for merge which also upgrades the severity into Warning back, we need to turn off the analyzer on test projects before merge because it would cause a warning for valid scenarios like:

runtime/src/libraries/System.Linq/tests/CastTests.cs

Line 31 in 319fcd9

Assert.Throws<InvalidCastException>(() => { foreach (var t in rst) ; });

and

runtime/src/libraries/System.Linq/tests/OfTypeTests.cs

Lines 42 to 43 in 319fcd9

	int[] source = { 99, 45, 81 };
	Assert.Empty(source.OfType<long>());

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Issue Details

---

CA2021: Do not call Enumerable.Cast<T> or Enumerable.OfType<T> with incompatible types is a new analyzer that added in January.

Because it is Warns by default it caused build failure on automation PR that updates analyzer feed which caused by some edge case scenario bug.

Because the bug could cause a failure in other repos we lowered the severity of the analyzer until those bugs get fixed. Now the fix is ready for merge which also upgrades the severity into Warning back, we need to turn off the analyzer on test projects before merge because it would cause a warning for valid scenarios like:

runtime/src/libraries/System.Linq/tests/CastTests.cs

Line 31 in 319fcd9

Assert.Throws<InvalidCastException>(() => { foreach (var t in rst) ; });

and

runtime/src/libraries/System.Linq/tests/OfTypeTests.cs

Lines 42 to 43 in 319fcd9

	int[] source = { 99, 45, 81 };
	Assert.Empty(source.OfType<long>());

Author:	buyaa-n
Assignees:	-
Labels:	area-Meta
Milestone:	-

[stephentoub]

LGTM if CI is green

[buyaa-n]

The only failure is unrelated and known