Rename Slack channels for clarity and convention

For the sake of clarity (and my Slack sidebar), thought it would be beneficial to update a few Support and Infrastructure-related Slack channels so that they align with convention.

Current name	Proposed name	DRI/Owner
spt-hiring	spt_hiring	See #2244 (comment 318618455)
support-team-chat	support_team-chat
sentry	feed_sentry
sentry-staging	feed_sentry-staging
gitlab-issues-feed	feed_gitlab-issues	See #2244 (comment 327429723)
announcements	feed_announcements	See #2244 (comment 337560593). Removing from scope of this issue to complete MRs.
alerts-general	feed_alerts-general
alerts	feed_alerts
zd-gitlab-com-feed	feed_zd-gitlab-com
zd-self-managed-feed	feed_zd-self-managed
zd-federal-feed	feed_zd-federal
support-managers	support_managers
support-bootcamp-geo	support_bootcamp-geo
support-bootcamp-omnibus	support_bootcamp-omnibus
spt-k8s-bootcamp	support_bootcamp-k8s
support-bootcamp-ha	support_bootcamp-ha

Per Amber L., issue can be created in https://gitlab.com/gitlab-com/business-ops/itops/issue-tracker/issues.