[7.x] [logstash] remove "upgrade" functionality now that .logstash is a system index (#87056)

[spalger]

Backports the following commits to 7.x:

• [logstash] remove "upgrade" functionality now that .logstash is a system index ([logstash] remove "upgrade" functionality now that .logstash is a system index #87056)

[kibanamachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: 1e4784d

Metrics [docs]

Module Count

Fewer modules leads to a faster build time

id	before	after	diff
logstash	61	54	-7

Async chunks

Total size of all lazy-loaded chunks that will be downloaded as the user navigates the app

id	before	after	diff
logstash	60.8KB	55.9KB	-4.9KB

Distributable file count

id	before	after	diff
default	47595	48356	+761

Page load bundle

Size of the bundles that are downloaded on every page load. Target size is below 100kb

id	before	after	diff
logstash	25.9KB	25.6KB	-313.0B

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream