Inconsistency: Strobe only encodes individual lane validity at C≥8

## Issue

https://abs-tudelft.github.io/tydi/specification/physical.html#signals

Mentions that `strb` encoding individual lane validity is contingent on C≥8. This is repeated in a few other places.

However, https://abs-tudelft.github.io/tydi/specification/physical.html#signal-omission makes `strb` contingent on C≥7.
And https://abs-tudelft.github.io/tydi/specification/physical.html#complexity-c likewise suggests that "The indices of the active data lanes can be described with a simple range." only applies to C<7.

## Assumed/Suggested Fix

`strb` encoding individual lane validity/activity should be contingent on C≥7. As otherwise, C=7 is identical in functionality to C=6.
Instead, the intent was most likely to have C≥7 add support for individual lane validity (a `strb` bit per element lane), while C≥8 adds support for individual lane-based sequence terminations (a `last` signal per element lane).