Bump alertmanager to v0.34.0 (19-2-stable)

What does this MR do?

Backport of !9718 (merged) to 19-2-stable.

Bump alertmanager to v0.34.0

• Updates alertmanager from version 0.31.1 to version 0.34.0.
• Updates github.com/hashicorp/memberlist version in dependency decisions support file.
• Enables go build to complete without Elm installed.

Related https://gitlab.com/gitlab-org/omnibus-gitlab/-/work_items/10055

Changelog: changed

Signed-off-by: Robert Marshall rmarshall@gitlab.com (cherry picked from commit 13f36256)

Related issues

Relates to https://gitlab.com/gitlab-org/omnibus-gitlab/-/work_items/10055

Checklist

• MR title and description are up to date, accurate, and descriptive.
• MR targeting the appropriate branch.
• Latest Merge Result pipeline is green.
• The manual Trigger:ee-package jobs have a green pipeline running against latest commit.
• build-package-on-all-os job within the Trigger:ee-package downstream pipeline succeeded.