Selecting XML nodes with XPath: retarget net10.0, drop null-forgiving operators, add LINQ to XML XPath method

[vladimir-pecanac-main]

Updates the sample for the article "Selecting Xml Nodes With XPath in C#".

• Retargets both projects from net7.0 (out of support since 14 May 2024) to net10.0.
• Test packages: Microsoft.NET.Test.Sdk 18.10.0, xunit 2.9.3, xunit.runner.visualstudio 4.0.0, coverlet.collector 10.0.1.
• Program.cs loads the catalog through Path.Combine(AppContext.BaseDirectory, "BooksCatalog.xml"), so "dotnet run" works from the project folder instead of throwing FileNotFoundException. It also prints a message rather than an empty line when the single-node query finds nothing.
• XmlNodesSelector.cs drops all four null-forgiving (!) operators: SelectSingleBook() returns string? and null-checks, the two list methods return [] when SelectNodes() returns null.
• Adds SelectBooksWithLinqToXml(XDocument) using XPathSelectElements(), the LINQ to XML equivalent of SelectBooks().
• Four new tests: SelectSingleNode returns null on no match, SelectNodes returns an empty list on no match, an unprefixed expression returns 3 of the 4 books (the fourth carries a default namespace), and the LINQ to XML method returns the same books as SelectNodes().
• The three existing assertions compare multi-line XElement.ToString() output against raw string literals, so they call ReplaceLineEndings() on both sides. Without it the tests fail on a clone where git leaves the LF blob unconverted while XElement.ToString() emits CRLF.

Build and tests on SDK 10.0.302 / runtime 10.0.10: 0 warnings, 0 errors, 7/7 passing.