docs(v2): improve v1->v2 migration docs

[aleks-p]

helm upgrade --reuse-values will fail if the new chart has features that rely on values bundled with the chart (e.g. failed for me because of how #5067 and #4983 are set up).

Hence we switch to --reset-then-reuse-values which will overlay the installation values on top of the chart-bundled defaults.

Also updating the target chart version to 2.0.0 which is not yet released. We might want to backport this, but it is probably fine if we don't.

---

Note

Low Risk
Docs-only changes with no runtime impact, though pinning to an unreleased chart version could confuse users if they follow the commands verbatim.

Overview
Adjusts the v1→v2 storage migration guide to recommend helm upgrade with --reset-then-reuse-values (instead of --reuse-values) so chart-default values are retained while overlaying existing install values.

Pins the single-binary migration commands (Phase 1 dual ingest and Phase 3 cutover) to grafana/pyroscope --version 2.0.0, and removes the prerequisite callout about persistence being enabled.

^{Reviewed by Cursor Bugbot for commit 37f2e12. Bugbot is set up for automated code reviews on this repo. Configure here.}

[aleks-p]

I think this causes more confusion than anything. If a user tries to set it globally all components will request a volume, some will not get one and the installation will hang.