chore(ci): use stencil nightly tags (#26927)

this commit removes the retrieval of stencil source code and building
the library from source. with
https://github.com/ionic-team/stencil/commit/24887c0da0432a67f4c663063454d2a68fc19fa7
(https://github.com/ionic-team/stencil/pull/4100), stencil now creates
nightly builds from `main@HEAD` of the repository. these pre-built
packages are tagged as "nightly", allowing us to bypass the previously
mentioned build steps and install the package directly

rename stencil-eval to stencil-nightly to better reflect what the
workflow is running. since ionic framework v7.0 has been released,
its feature branch has been removed/merged into `main`. remove
usages of `feature-7.0` in favor of `main`