Use String.Contains(char) instead of String.Contains(String)

[Youssef1313]

Similar to one of the changes in #36304

[stephentoub]

@Youssef1313, I appreciate your interest in contributing, but a bunch of these changes are to tests or tools where it really doesn't matter which overload is used, and others are to source (some src, some tests) that builds against versions of .NET that don't have the relevant overload, so it's going to fail to build.

[Youssef1313]

@Youssef1313, I appreciate your interest in contributing, but a bunch of these changes are to tests or tools where it really doesn't matter which overload is used, and others are to product src that builds against versions of .NET that don't have the relevant overload, so it's going to fail to build.

If not all of the changes doesn't matter, I can update the PR to exclude only the files that belong to tests and where it doesn't matter.

If it doesn't matter in all of them, you can close the PR.

[stephentoub]

If you'd like to remove the cases that fail to compile, we can see what's left. From a quick skim, I'm expecting most of the changes will end up getting reverted.

[stephentoub]

@Youssef1313, given it's been a few weeks, I'm going to go ahead and close this. If you'd like to submit a PR that just handles such changes for product code under libraries, we can try to get that in. Thanks for your interest.