Remove tempo serverless

[electron0zero]

What this PR does:
serverless was deprecated in tempo 2.7 release.

Tempo serverless related code, config options, and docs is being removed.

This PR Removes:

1. serverless config options from the tempo config
   2.serverless doc pages and mention from the docs
2. serverless dashboard panels from tempo-mixin, and removes mention of serverless from playbook

following config options are no longer valid:

querier:
    search:
        prefer_self: <int>
        external_hedge_requests_at: <duration>
        external_hedge_requests_up_to:  <duration>
        external_backend: <string>
        google_cloud_run: <string>
        external_endpoints: <array>

following serverless related metrics are being removed:

tempo_querier_external_endpoint_duration_seconds
tempo_querier_external_endpoint_hedged_roundtrips_total
tempo_feature_enabled

Which issue(s) this PR fixes:
Fixes #4014

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[mdisibio]

👋 Good bye serverless. Nice cleanup

[joe-elliott]

o7 serverless

I'm good on this as well. Let's just wait for Kim to weigh in on removing the docs pages and then we can merge.

[knylander-grafana]

Thank you for updating the doc! Changes look good! We should add an alias for any deleted pages. For an example of an alias: #4598 (comment)

Note that aliases use relative locations. For detailed instructions: https://grafana.com/docs/writers-toolkit/write/front-matter/#aliases