Switch to GitHub Actions workflow for deploy to WordPress.org

**Is your feature request related to a problem? Please describe.**

Having to run `npm install`, `npm run build`, `npm run deploy` as wrappers of a Grunt dependency (and Grunt-extension dependencies), which involves cleaning directories, copying files, with exclusions, and remembering to do it after a release on GitHub, seems like extra work and effort.

**Describe the solution you'd like**
With a one-time switch to a [commonly-used custom action for deploying to WPORG](https://github.com/10up/action-wordpress-plugin-deploy), this task can be handled automatically on each GitHub release.

Here's a starting point:

```yml
name: Deploy to WordPress.org
on:
release:
types: [released]
# Allow manual triggering of the workflow.
workflow_dispatch:

permissions: {}

jobs:
release:
name: New release to WordPress.org
runs-on: ubuntu-latest
steps:
- name: Checkout
uses: actions/checkout@v4
with:
persist-credentials: false

- name: Install SVN (Subversion)
run: |
sudo apt-get update
sudo apt-get install subversion

- name: Push to WordPress.org
uses: 10up/action-wordpress-plugin-deploy@54bd289b8525fd23a5c365ec369185f2966529c2 # 2.3.0
env:
SLUG: polldaddy
SVN_PASSWORD: ${{ secrets.SVN_PASSWORD }}
SVN_USERNAME: ${{ secrets.SVN_USERNAME }}
BUILD_DIR: tmp/build
```

You'll notice that this includes a build directory config at the bottom - by using a `.deployignore` file to contain the [exclusions](https://github.com/Automattic/crowdsignal-plugin/blob/2771eae9dbdbc64576ff1b32ae020c388d868c81/Gruntfile.js#L20-L32) then this "copy the things we actually want to deploy to a new folder" step can be dropped.

Read the existing deployment exclusions in Gruntfile.js lines 20-32 and compare them with the proposed .deployignore approach. Check the GitHub Actions and action-wordpress-plugin-deploy documentation, then verify that a release and manual workflow can deploy with the stated SVN secrets and that the resulting package contains only the intended files.

Written by the indexing model from the issue text.