Feature: client-admin upgrade to react router v6

[thomassth]

npm audit --prod should now report no vulns.

[MaanasArora]

@thomassth I cloned the project and tried to validate it!

I see only 1 high-severity vulnerability, perhaps because some modules were updated after this PR was made:

path-to-regexp  0.2.0 - 1.8.0
Severity: high
path-to-regexp outputs backtracking regular expressions - https://github.com/advisories/GHSA-9wv6-86v2-598j
fix available via `npm audit fix`
node_modules/react-router/node_modules/path-to-regexp

I've updated express in a branch on my fork that fixes this issue, though I'm not sure if we're willing to update express for backward compatibility (and not tested the implementation fully).

Looks good to me otherwise!