Background events are now sent in the `Background` category, rather
than `backend`. Conventionally we use the term "background" over
"backend", as it's not really a "backend" in the normal sense since
it's a client background process. Also it's capitalized because all of
the other event categories are capitalized as well.

The metrics URL has been updated to use `background` instead of
`backend` as well, for consistency.

Luckily we don't have to worry about our metrics being disjointed due
to this name change, because the background metrics never worked to
begin with! So there will be none under the old name. The metrics will
be made functional in a separate PR.

* origin/master: (44 commits)
  Add category in eventOpts (#9164)
  Update changelog for v8.0.7 (#9161)
  Version v8.0.7
  Remove web3 e2e tests (#9159)
  Add web3 usage metrics, prepare for web3 removal (#9144)
  Use `pathname` instead of URL for `currentPath` metrics parameter (#9158)
  Remove `url` parameter from `metricsEvent` (#9157)
  Change MetaMetrics category for background events (#9155)
  remove .network-name height
  Use luxon@1.24.1 (#9154)
  Update 'react-devtools' to ^4.8.0 (#9140)
  Fix connection removal bug (#9137)
  Add source map validator to CI (#9135)
  Update source map validator target files (#9133)
  Improve sourcemap validator console report (#9131)
  Add `validate-source-maps` npm script (#9134)
  Non-zero exit code upon failure to validate source maps (#9132)
  Update `brfs` from v1.6.1 to v2.0.2 (#9115)
  Factor out `getEnvironment` function in build script (#9114)
  Update `browserify` from v16.2.3 to v16.5.1 (#9113)
  ...