promtail: update promtail base image to debian:bullseye-slim

[lizzzcai]

What this PR does / why we need it:
Update the base image of promtail to keep the image secure.
From debian:buster-slim to debian:bullseye-slim
Which issue(s) this PR fixes:
Fixes #4140

Special notes for your reviewer:
docker scan report before the update:

Package manager:   deb
Project name:      docker-image|grafana/promtail
Docker image:      grafana/promtail:update-promtail-base-image-517ba7c
Platform:          linux/amd64
Base image:        debian:10.11-slim

Tested 89 dependencies for known vulnerabilities, found 62 vulnerabilities.

Base Image         Vulnerabilities  Severity
debian:10.11-slim  60               2 critical, 8 high, 6 medium, 44 low

Recommendations for base image upgrade:

Major upgrades
Base Image      Vulnerabilities  Severity
debian:11-slim  35               1 critical, 0 high, 1 medium, 33 low

docker scan report after the update:

Package manager:   deb
Project name:      docker-image|grafana/promtail
Docker image:      grafana/promtail:update-promtail-base-image-517ba7c-WIP
Platform:          linux/amd64
Base image:        debian:11.1-slim

Tested 100 dependencies for known vulnerabilities, found 35 vulnerabilities.

According to our scan, you are currently using the most secure version of the selected base image

Checklist

• Documentation added
• Tests updated

[CLAassistant]

All committers have signed the CLA.