Refactor the upgrading from source docs

What does this MR do?

Instead of requiring one separate document for every version (which is created by release managers), we now use a single document for both Community Edition and Enterprise Edition. This allows developers to add guidelines right away, instead of release managers having to determine what to add.

Version specific upgrading instructions are placed in a separate section. This prevents instructions from lingering around for many versions should a developer forget to remove them. This also ensures that all instructions are kept in a single place, instead of being spread across different documents.

A minor downside is that CE and EE guidelines now live in the same document, which could cause merge conflicts. Since we are working towards a single codebase, this should only be an issue until we merge the codebases together; something we expect to do in the coming months.

This commit also removes all old upgrading instructions, but these can still be accessed using Git branch specific URLs (included in the updating README).

Related issues

Closes https://gitlab.com/gitlab-org/release/framework/issues/190

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9519

Review checklist

• Your team's review (required): @marin
• Technical writer's review (required): @axil / @marcia
• Merge the EE-MR first, CE-MR afterwards