Fixes dotnet/runtime#78587

Flags 3 cases in dotnet/runtime, all of them in projects where we've avoided making the switch because they multi-target to older TFMs (SearchValues is a .NET 8+ API).

The analyzer flags cases like

const string MyValues = "abcdef";
readonly char[] MyValues = new[] { 'a', 'b', 'c', 'd', 'e', 'f' };

span.IndexOfAny("abcdef");
span.IndexOfAny("abcdef"u8);
span.IndexOfAny(MyValues);
span.IndexOfAny(new[] { 'a', 'b', 'c', 'd', 'e', 'f' });
string.IndexOfAny("abc".ToCharArray());

for all {Last}IndexOfAny{Except} and ContainsAny{Except} overloads for byte/char and suggests that they be rewritten as

private static readonly SearchValues<char> s_myChars = SearchValues.Create("abcdef");

span.IndexOfAny(s_myChars);

We'll also rewrite the string.IndexOfAny(char[]) overload to use string.AsSpan().IndexOfAny(SearchValues) instead.
We won't do the same for IndexOfAny(char[], int) or IndexOfAny(char[], int, int) as they would require us to inject more logic to account for the differences in behavior (returning offset from start vs. from 0)

If the values are specified by a constant in an appropriate scope, we'll reuse said constant to feed the SearchValues.Create.

If we're not using the original member and there are now no other uses, we'll remove it.

For a full list of patterns that we do/don't recognize, see this test source.

Potential improvements:

• IDE0300 will suggest replacing new[] { 'a', 'b', 'c' } with ['a', 'b', 'c'], whereas this analyzer does not recognize collection literals.