Introduce showInDashboard instead of showIn

[VincentLanglet]

Subject

Closes #7642.

Changelog

### Added
- `AdminInterface::showInDashboard()` method.

### Deprecated
- `AdminInterface::showIn()` method.

[soullivaneuh]

I am unsure about the final naming, however this looks to fix the issue.

However, be aware that the issue is reported on the 3.x release. Can this be easily back ported?

[VincentLanglet]

I am unsure about the final naming

I'm open to suggestion.
The call showIn was made on the Context_Dashboard constant so showInDashboard seems natural to me.
But I didn't look where it's use so maybe another name is better.

However, be aware that the issue is reported on the 3.x release. Can this be easily back ported?

In 3.x, it's "only" a deprecation, which could be ignored.
Not sure it worth back porting this (and dealing with conflict).
When bumping to 3.x to 4.x, you'll just have to rename showIn to showInDashboard.

[SonataCI]

Could you please rebase your PR and fix merge conflicts?