Remove references to webapp in the docs

[ian-at-airbyte]

What

This PR updates documentation to reflect the removal of the airbyte-webapp component. It appeared in a few random places throughout the docs.

A guide already exists to explain ingress updates as a result of work done for 1.7, so there was little left to do here.

How

• Searched for instances of webapp, server, and related environment variables to see how they were discussed.

• Removed or updated these instances, as required. There are places where I've not changed anything because it didn't make sense to do so.

  For example, the chart v2 migration guide already explains to use global.airbyteUrl in your values file, so it was necessary to keep the original WEBAPP_URL variable in the text so they knew what not to do.

• Updated some various terminal outputs to update older version numbers to 1.8.0. Not specifically related to this, but good to do from time to time.

Review guide

There is no particular reading order. Most changes were just a line or two in sundry files throughout the docs site.

User Impact

Minimal.

Can this PR be safely reverted and rolled back?

• YES 💚
• NO ❌

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