Consolidate from source upgrade notes

What does this MR do?

A quick iteration on the consolidation of all version specific upgrade notes. Move from-source notes for three versions into the main upgrades/index.md

Additionally.

13.0.1 > 13.1

the rack attack change was made in 13.1, not 13.0.1

It wasn't backported.

References:

• bb680fc5
• https://gitlab.com/gitlab-org/gitlab/-/blob/v13.0.14-ee/config/initializers/rack_attack_new.rb (present)
• https://gitlab.com/gitlab-org/gitlab/-/blob/v13.0.14-ee/config/initializers/rack_attack.rb (not present)
• https://gitlab.com/gitlab-org/gitlab/-/blob/v13.1.0-ee/config/initializers/rack_attack.rb (present)
• https://gitlab.com/gitlab-org/gitlab/-/blob/v13.1.0-ee/config/initializers/rack_attack_new.rb (not present)

Version specific changes

For consistency with the planned change in the epic

Rename #version-specific-upgrading-instructions to #version-specific-changes

Related issues

relates to this specific thread: &9581 (comment 1229951893)

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