Update the README.md with more specific updating steps

[hbokh]

Summary

The current README.md is having a different update strategy than suggested in multiple forum posts.
@BYK has answered multiple questions in the Sentry forum that contradicts this specific part in README.md:

Updating Sentry:
...
We recommend (and sometimes require) you to upgrade Sentry one version at a time. That means if you are running 20.6.0, instead of going directly to 20.8.0, first go through 20.7.0. Skipping versions would work most of the time, but there will be times that we require you to stop at specific versions to ensure essential data migrations along the way.

FYI: I ran an upgrade from Sentry 9.0.0 to 9.1.2 --> 20.12.1 --> 21.5.1.
But it looks like I could have skipped the 20.12.1 in the steps.

Motivation

It is unclear from the README.md in this repo that a Sentry on premise upgrade can be done from 9.1.2 directly to 21.5.1.

Additional Context

Examples from the Sentry forum to illustrate my motivation:

• I’d try upgrading directly and running migrations at each step. You should be able to do 9.0.0 -> 9.1.2 -> 20.12.1
  https://forum.sentry.io/t/the-best-way-to-upgrade-to-latest-sentry-version/12275/2
• Right now it is safe and recommended to go directly to 21.4
  https://forum.sentry.io/t/upgrade-9-1-2-current-via-export-import/13695/4
• Yup, right now you can go directly to 21.5.1 which is the latest version.
  https://forum.sentry.io/t/best-way-to-upgrade-to-latest-sentry-version-from-v9-0-0/11418/4

[BYK]

Thanks a lot for bringing this up @hbokh and sorry for the confusion! Even the current wording there is a bit confusing.

Do you have any suggestions do make it clearer (can be a PR to get your name on it) or you wanna roll with whatever we come up with? 🙂

[BYK]

Even more, we have this: https://develop.sentry.dev/self-hosted/#upgrading

Maybe this is the correct opportunity to close out #790?

[hbokh]

Do you have any suggestions do make it clearer (can be a PR to get your name on it) or you wanna roll with whatever we come up with? 🙂

Thanks Burak!
My suggestion is to at least update the Updating Sentry block that it is not really needed anymore to upgrade Sentry one version at a time.
If you at Sentry have the experience that going from 9.1.2 to 21.5.1 is going to be fine for (let's say) 90% of all cases, I'd say that should be shown instead.
(It might also be a good addition to state there is a sufficient amount of RAM avaliable, even for developing or only testing upgrades: using less than the advised amount of 8GB RAM is recipe for failure.)

As for #790: I personally would shorten the README.md in this repo and point directly to your docs instead.

For the rest: roll with whatever you come up with ! 😇