Update MV3 migration advice

[rebloor]

Implement the following updates:

• updates advice regarding host permission.
• add information about replacing extension.lastError with runtime.lastError and extension.getURL with runtime.getURL.

[Rob--W]

Currently missing from documentation:

• fetch and XHR in content scripts no longer allow cross-origin requests through extension permissions. Instead, the destination server should use CORS, or the extension has to send a message to go the background page (or an extension tab or frame) instead, to ask it to make a request on behalf of the content script. This changed in https://bugzilla.mozilla.org/show_bug.cgi?id=1578405
• userScripts API of MV2 differs from userScripts API in MV3 (maybe one small note in the scripting section? Also emphasize that the AMO only approves of the use of the API for user script managers only)
• CSP section should also mention that it now includes upgrade-insecure-requests by default, see https://bugzilla.mozilla.org/show_bug.cgi?id=1797086

Although not specific to MV3, should we mention the data_collection_permissions requirement for visibility? (While emphasizing that it is also needed in MV2). I haven't put too much thought into it, the question just crossed my mind.

[rebloor]

@Rob--W I'd also wondered about data_collection_permissions. On the basis that someone migrating to Manifest V3 has most likely implemented the data collection experience in their MV2 extension and as that will probably be an existing extension won't be required to use this feature, I've added this as a note at the end of the checklist.