Refactor: replace usages of "starterBlueprints" with "starterMods"

[mnholtz]

Follow-up of https://github.com/pixiebrix/pixiebrix-extension/pull/5978

In order to replace all usages of the term starterBlueprint, we need to change the background script messenger method installStarterBlueprints. This causes a backwards-incompatible change.

We opted to initially not do a partial refactor, i.e. to have a mix of things called starterBlueprints and starterMods, because we felt that this would make for a better developer experience over all (in terms of e.g. greping for one term or the other).

Options:

• Do the full refactor, which would require two PRs in the extension and app repo, and a check for the right extension version number on the app-side to call the correct method
• Do a partial refactor, preserving the messenger method name installStarterBlueprints but renaming everything else