feat: update openfaas-loki to use new OCI repository

[LucasRoesler]

Description

Update the install configuration and process for openfaas-loki so that it uses the OCI repository hosted by Github

Motivation and Context

• I have raised an issue to propose this change, which has been given a label of design/approved by a maintainer (required)

How Has This Been Tested?

Tested locally with kind

$ kind create cluster
$ arkade install openfaas-loki
$ helm list 
NAME            NAMESPACE       REVISION        UPDATED                                     STATUS          CHART                   APP VERSION
openfaas-loki   default         2               2023-09-30 19:01:45.753063527 +0200 CEST    deployed        openfaas-loki-1.7.3     v1.7.3 

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Documentation

• I have updated the list of tools in README.md if (required) with ./arkade get --format markdown
• I have updated the list of apps in README.md if (required) with ./arkade install --help

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have signed-off my commits with git commit -s

• I have tested this on arm, or have added code to prevent deployment

[alexellis]

Approved