Mend has a page about this CVE-2019-0820 issue, explaining when this notice might also be a false positive.
For your situation it is a false positive
you are on .NET 6, which is higher than the mentioned vulnerable runtimes (see below) - your WebApp.proj has <TargetFramework>net6.0</TargetFramework>
you reference System.Text.RegularExpressions version 4.3.1 or even higher.
From that page:
In case the project has the CVE-2019-0820 reported then if it is actually vulnerable or not depends on the used .NET Core Runtime version.
The vulnerable .NET Core Runtime versions for this CVE are:
v1.0 branch: 1.0 - 1.0.16 (exclusive)
v1.1 branch: 1.1 - 1.1.13 (exclusive)
v2.1 branch: 2.1 - 2.1.11 (exclusive)
v2.2 branch: 2.2 - 2.2.5 (exclusive)
If the user is running a higher .NET core version than listed above while using the package version 4.3.1 then they are secured from the vulnerability and they can safely ignore the alert.