New rule S6930: Backslash should be avoided in route templates

[antonioaversa]

RSPEC: https://github.com/SonarSource/rspec/pull/3662

PRs:

• S6930: Support for ASP.NET 4x #8897
• S6930: Support for ASP.NET Core 2.x #8900
• S6930: Support for ASP.NET Core 3 and above #8901

Leftovers moved to #8926