[Gatsby-Plugin] Set Browser Agent to load via CDN so it always uses the latest version

[rudouglas]

Freddy has already done work on including the agent via CDN, which means the plugin is always up to date with the latest agent version here. This is installable using npm i gatsby-plugin-newrelic@alpha which I tested out and it looks like it's working as expected but may need some further testing to ensure we're getting all the same data.

The only change to be aware of is the instrumentationType: proAndSPA is now instrumentationType: spa so this will need to be changed in each sites browser-config.js were appropriate

• [ ] In depth testing to make sure we are getting the correct Browser data and there are no unforeseen issues
• [ ] Merge the PR into main
• [ ] Bump the gatsby-plugin-newrelic version in gatsby-theme-newrelic to the latest
• [ ] Cut a release of gatsby-theme-newrelic
• [ ] Bump each site's version of gatsby-theme-newrelic (Developer, Docs, OpenSource)
  • [ ] Change references of proAndSpa to spa in the sites gatsby-config.js for the gatsby-theme-newrelic plugin, for example here

[jpvajda]

We discussed if we really want to automatically install the latest browser agent and if that won't cause us problems.

[jpvajda]

de-coupling from our work to upgrade this plugin as isn't not required, so we can look at this later.