chore: add Made for UDS badge to Mattermost

[Racer159]

Description

This adds the Made for UDS badge to Mattermost given it meets the following:

Integrations

Istio

• - Must define any external interfaces under the expose key.
• - Must deploy and operate successfully with Istio injection enabled in the namespace.
• - Should avoid workarounds such as disabling strict mTLS peer authentication.

Network Policies

• - Must define network policies under the allow key as required.
• - Should minimize network policies to specific selectors needed for Ingress/Egress traffic.
• - May template network policy keys to provide flexibility for delivery customers to configure.

Keycloak

• - Must use and create a Keycloak client through the sso key if the application provides a user login.
• - Should consider security options during implementation to provide the most secure default possible (i.e. SAML w/SCIM vs OIDC).
• - Should name the client <App> Login (i.e. Mattermost Login) to provide login UX consistency.
• - Should clearly mark the client id with the group and app name uds-<group>-<application> (i.e. uds-swf-mattermost) to provide consistency in the Keycloak UI.
• - May end any generated secrets with -sso to easily locate them when querying the cluster.
• - May template Keycloak fields to provide flexibility for delivery customers to configure.

Prometheus

• - Must implement monitors for each application metrics endpoint using it's built-in chart monitors, the Package CR monitor key, or manual monitors in the config chart.

Exemptions

• - Must minimize the scope and number of the exemptions to only what is absolutely required by the application
• - Must have documented rationale for any exemptions present

Structure

• - Should expose all configuration (uds.dev CRs, additional Secrets/ConfigMaps, etc) through a Helm chart (ideally in a chart or charts directory).
• - Should limit the use of Zarf variable templates and prioritize configuring packages via Helm value overrides
• - Should implement or allow for multiple flavors (ideally with common definitions in a common directory)

Testing

• - Must implement Journey Testing to cover the basic user flows and features of the application, especially where an application interacts with an external service / interface.
• - Must implement Upgrade Testing to ensure that the current development package works when deployed over the previously released one.
• - Should lint their configurations with appropriate tooling such as yamllint and zarf dev lint.

Maintenance

• - Must have a dependency management bot (such as renovate) configured to open PRs to update core package and support dependencies.
• - Must release its package to the ghcr.io/defenseunicorns/packages/<group> namespace as the application's name (i.e. ghcr.io/defenseunicorns/packages/uds/mattermost)

General

• - Must be capable of operating within an internet-disconnected (air-gapped) environment
• - Must be maintained by a resourced team that is explicitly defined as maintaining the project (i.e. in CODEOWNERS)

Related Issue

Fixes #N/A

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Other (security config, docs update, etc)

Checklist before merging

• Test, docs, adr added or updated as needed
• Contributor Guide Steps followed