Remove overridden puma.service.erb template

We remove our custom puma.service.erb template located at
lib/capistrano/templates, as the upstream capistrano3-puma gem (v6.2.0)
now supports all the adjustments we previously needed.

This template was originally copied and modified in
PR #5842 (Bump capistrano3-puma from 5.2.0 to 6.0.0),
with the following relevant changes:

1. WatchdogSec=0
We disabled systemd's watchdog feature by setting WatchdogSec=0.
This is now covered upstream since v6.1.0 via:
> <% if fetch(:puma_systemd_watchdog_sec) && fetch(:puma_systemd_watchdog_sec) > 0 %>
> WatchdogSec=<%= fetch(:puma_systemd_watchdog_sec) %>
> <% end %>

To preserve the same behavior, we now explicitly set:
> set :puma_systemd_watchdog_sec, 0
in config/deploy.rb, instead of relying on the template override.

2. SyslogIdentifier=puma
In our custom template, we replaced the default line:
> SyslogIdentifier=<%= fetch(:puma_service_unit_name) %>
with:
> SyslogIdentifier=puma

This was done to avoid introducing a new config variable
in the installer. However, now that we want to remove the overridden template,
we must accept the default behavior from the gem:
> set :puma_service_unit_name, -> { "puma_#{fetch(:application)}_#{fetch(:stage)}" }

As a result, the generated SyslogIdentifier will change from "puma" to something
like "puma_consul_staging".

The only implication is that we'll open a PR in the installer
to replace the hardcoded "puma" with the dynamic default,
so everything stays consistent.

Another option would be to override puma_service_unit_name
with "puma" to keep the SyslogIdentifier exactly as before.
However, this would also affect the names of the systemd services and sockets
(e.g., puma.service instead of puma_consul_staging.service),
which may lead to conflicts or require manual cleanup steps like disabling
and removing the old units. For now, we prefer to avoid that.

3. After=network.target
We had removed the "syslog.target" from the "After=" directive,
as we didn't depend on syslog.

However, keeping "After=syslog.target network.target" does not negatively impact us,
since Puma logs to files directly using StandardOutput=append:....
So we can also adopt the default value from template here.

4. ExecStart
In version 6.2.0, a small change was made to the template logic for the ExecStart line.
This change does not affect us. Since we don't define puma_use_login_shell,
the behavior remains exactly the same as before.

With these changes already supported in the gem, we no longer need
a local copy of the puma service template.

config/deploy.rb

@@ -56,6 +56,7 @@ set :puma_service_unit_name, -> { "puma_#{fetch(:application)}_#{fetch(:stage)}"
 set :puma_bind, "unix://#{shared_path}/tmp/sockets/puma.sock"
 set :puma_access_log, -> { File.join(shared_path, "log", "puma_access.log") }
 set :puma_error_log, -> { File.join(shared_path, "log", "puma_error.log") }
+set :puma_systemd_watchdog_sec, 0
 
 set :delayed_job_workers, 2
 set :delayed_job_roles, :background