RM dashboard to use string for sha and version

The release management dashboard was not specifying the unit type for displaying the sha and version in the first few panels so it was defaulting to Number. We ran into a situation where the sha was something like 18e41223786, which caused it to be recognized as an exponential number ((18)10^41223786) and displayed as infinity:

This MR explicitly sets the unit to String to prevent that from happening.

Before	After
Inspecting the panel JSON shows "unit": "none"	Inspecting the panel JSON shows "unit": "String"

Snapshot dashboard: https://dashboards.gitlab.net/dashboard/snapshot/yCvppdi8cvSaJlmgccSlSxzzDPngijIy