Remove Browsersync for Eleventy v2.x compatibility

[pkvach]

A compatibility issue with Eleventy v2.x came up after a recent merge request "Bump axios and @11ty/eleventy #180". It updated Eleventy from 1.0.0 to 2.0.1
https://github.com/google/eleventy-high-performance-blog/actions/runs/6827178225/job/18568727206

In Eleventy v2 Eleventy Dev Server replaces Browsersync (11ty/eleventy#1305).

So I removed the Browsersync related code.
Eleventy Dev Server has built-in support for 404.html.
And there is no need for additional CSP code.

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[cramforce]

Is that gonna work with 11ty 1.0?

[pkvach]

This won't work for Eleventy v1.0 because Eleventy Dev Server was added in v2.0
https://www.11ty.dev/blog/eleventy-v2/#plugins

[cramforce]

Should we update the package.json dependency on 11ty?

[pkvach]

I noticed that the 11ty version has already been updated in the package.json file in pull request #180.
The update has already been merged, but the build failed:
https://github.com/google/eleventy-high-performance-blog/actions/runs/6827178225/job/18568727206

[cramforce]

OK, let's get this merged. I lost the ability to configure CI when I left Google, so stuff needs to be validated on a fork.

[pkvach]

Sure, I've already checked these changes. Thank you.