docs: simplify PostgreSQL setup by using 'postgresql' as release name (#18754)

Fixes #18751

Use `postgresql` as the Helm release name instead of `coder-db` to make
the service name more intuitive and eliminate confusion entirely.

## Changes
- Changed `helm install coder-db bitnami/postgresql` to `helm install
postgresql bitnami/postgresql`
- Updated PostgreSQL URLs from
`coder-db-postgresql.coder.svc.cluster.local` to
`postgresql.coder.svc.cluster.local`
- Removed explanatory notes about service naming (no longer needed)

## Benefits
✅ Makes examples work out-of-the-box for most users
✅ Uses the most straightforward and intuitive release name
✅ Eliminates confusion about service naming entirely
✅ Simpler documentation without complex explanations

## Testing
- Verified that `helm install postgresql bitnami/postgresql` creates
service named `postgresql`
- Confirmed this approach works with the connection URL
`postgresql.coder.svc.cluster.local`

Suggested by @EdwardAngert as a cleaner solution than explaining the
service naming dependency.

---------

Co-authored-by: blink-so[bot] <211532188+blink-so[bot]@users.noreply.github.com>
Co-authored-by: matifali <10648092+matifali@users.noreply.github.com>
Co-authored-by: EdwardAngert <17991901+EdwardAngert@users.noreply.github.com>
Co-authored-by: Edward Angert <EdwardAngert@users.noreply.github.com>

docs/install/kubernetes.md

@@ -44,7 +44,7 @@ on the internet that explain sensible configurations for this chart. Example:
 ```console
 # Install PostgreSQL
 helm repo add bitnami https://charts.bitnami.com/bitnami
-helm install coder-db bitnami/postgresql \
+helm install postgresql bitnami/postgresql \
     --namespace coder \
     --set auth.username=coder \
     --set auth.password=coder \
@@ -55,7 +55,7 @@ helm install coder-db bitnami/postgresql \
 The cluster-internal DB URL for the above database is:
 
 ```shell
-postgres://coder:coder@coder-db-postgresql.coder.svc.cluster.local:5432/coder?sslmode=disable
+postgres://coder:coder@postgresql.coder.svc.cluster.local:5432/coder?sslmode=disable
 ```
 
 You can optionally use the
@@ -69,7 +69,7 @@ self-managed PostgreSQL, the address will be:
 
 ```sh
 kubectl create secret generic coder-db-url -n coder \
-  --from-literal=url="postgres://coder:coder@coder-db-postgresql.coder.svc.cluster.local:5432/coder?sslmode=disable"
+  --from-literal=url="postgres://coder:coder@postgresql.coder.svc.cluster.local:5432/coder?sslmode=disable"
 ```
 
 ## 4. Install Coder with Helm