Update upgrade notes in CONTRIBUTING.md, README.md, values.yaml

[jessebot]

Description of the change

Adds a section about upgrades to clarify we should only do one major version at a time.

Benefits

just a little doc change to prevent gotchya :)

Possible drawbacks

open to feedback but I think this is minor enough that it's fine.

Applicable issues

• fixes Direct upgrade from 17.0 to 19.0 not supported, breaks existing installations #20
• fixes Feature: align chart versioning with nextcloud release version #499

Additional information

Checklist

• I have read the CONTRIBUTING.md doc.
• DCO has been signed off on the commit.
• Chart version bumped in Chart.yaml according to semver.
• (optional) Parameters are documented in the README.md

[jessebot]

I could improve this PR by also making a note in the values.yaml for the image.tag parameter as mentioned in #103 if you agree that's a good idea?

[provokateurin]

Upgrading to the latest minor version before upgrading to the next major version should probably also be documented: https://docs.nextcloud.com/server/latest/admin_manual/maintenance/upgrade.html#approaching-upgrades
This is not only for creating PRs to update the default version but also for the people who deploy the updates.

[provokateurin]

I could improve this PR by also making a note in the values.yaml for the image.tag parameter as mentioned in #103 if you agree that's a good idea?

Yeah that would be nice too, just to prevent people ending up in broken states...

[jessebot]

Wait, the docs are confusing, due to grammar:

Before you can upgrade to the next major release, Nextcloud upgrades to the latest point release.

If it's what you're saying here:

Upgrading to the latest minor version before upgrading to the next major version should probably also be documented

Then shouldn't it be this?:

Before you can upgrade to the next major release, you need to upgrade to the latest minor for your current major version.

If I can find the repo I need to change this in, I can go update that grammar 🤷

Happy to document that here as well though :)

[provokateurin]

Yes that sounds wrong indeed, you can fix it in https://github.com/nextcloud/documentation

[jessebot]

PR here :) nextcloud/documentation#12062