Not quite duplicated strings, mostly in metadata, e.g. [Obsolete] attributes

Examples:

* `git grep Obsolete src/ | grep NotSupported`
* `git grep Obsolete src/ | grep stub`

The compiler can share the same string in several locations - but it must be the exact same (sub)string.

The linker can also remove many of them. However the linker is not always used (e.g. interpreter) and this affect the platform assembly size (e.g. when loaded in memory by the compiler, IDE, mtouch...)

Recommended solution, using string constants for the repetitive messages. This way they can be shared and will consume less space in the platform assemblies, e.g.

```
const string ObsoleteStub = "Empty stub (not a public API).";

...

[Obsolete (Constants.ObsoleteStub)]
```

We could also had an URL for the most common patterns and document the issue in more details.

ref: https://github.com/xamarin/xamarin-macios/pull/9268