Upgrade Prometheus, Alertmanager and Thanos

[pracucci]

What this PR does:
To unblock #4085 we need to upgrade prometheus/common and Alertmanager which, due to dependencies, require to upgrade Prometheus, Thanos and some other dependencies. This is the reason why in this PR you will see a bunch of dependencies updated.

I've checked the changes in all core dependencies (AWS, GCP, Alertmanager, Prometheus, Thanos) and everything looks safe to me.

Which issue(s) this PR fixes:
N/A

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[pracucci]

My plan is to keep this, so that we'll not block this PR by Thanos. I will remove this replace once Thanos PR is merged.

[pstibrany]

Cortex changes LGTM.

[pracucci]

I've converted into a draft because, while reviewing thanos-io/thanos#3937, I've spot some issues which we need to fix first.

[pracucci]

I've converted into a draft because, while reviewing thanos-io/thanos#3937, I've spot some issues which we need to fix first.

Opened a Thanos PR thanos-io/thanos#4095

[pracucci]

I've converted into a draft because, while reviewing thanos-io/thanos#3937, I've spot some issues which we need to fix first.

Opened a Thanos PR thanos-io/thanos#4095

Merged and updated vendored Thanos in this PR. The risky change here is thanos-io/thanos#3937 but I've reviewed and, despite complex, I haven't noticed any issue (except for the race condition in the pool.BuckedBytes, which has been merged.