Remove special code for .NET 2.1- support

[bwateratmsft]

On August 21st, support for .NET 2.1 ends, so we can remove code that is needed to support it. For example, this:

vscode-docker/src/debugging/netcore/AspNetSslHelper.ts

Lines 137 to 152 in ffd43e4

	} else {
	// Otherwise try to manually edit the project file by adding a property group immediately after the <Project> tag
	// Allowing for leading and trailing whitespace, as well as XML attributes, this regex matches the <Project> tag as long as it is alone on its line
	const projectTagRegex = /^[ \t]*<Project.*>[ \t]*$/im;
	const matches = contents.match(projectTagRegex);
	if (matches && matches[0]) {
	// If found, add the new property group immediately after
	const propertyGroup = `
	<PropertyGroup>
	<UserSecretsId>${cryptoUtils.getRandomHexString(32)}</UserSecretsId>
	</PropertyGroup>`;
	const newContents = contents.replace(matches[0], matches[0] + propertyGroup);
	await fse.writeFile(projectFile, newContents);
	}
	}

.