Update .nvmrc to use latest LTS release (#220)

**Why**:

- Fix Federalist builds, which no longer support Node 10
- Avoid pinning to a specific version to reduce future maintenance overhead
- Use a version of Node which hasn't fallen outside the maintenance calendar, to avoid missing critical bug fixes or encountering other conflicts

See: https://nodejs.org/en/about/releases/

.nvmrc

@@ -1 +1 @@
-10
+lts/*