Update templates to replace `.Site.IsServer` with `hugo.IsServer`

[aj-stein-gsa]

Based on discussion with the team troubleshooting install in #68, we need to remove the deprecated template syntax causing the warnings reported with the currently required version of hugo. Output from the local environment on a developer's laptop.

% git checkout --track GSA/develop
% git rev-parse HEAD
020e3273a60f708f9d6b7be809fcad7f00f84f96
% git submodule update --init --recursive
% hugo mod get 
% hugo server -v --debug --minify
% hugo mod get
me@computer automate.fedramp.gov % hugo server -v --debug --minify
WARN  deprecated: --verbose was deprecated in Hugo v0.114.0 and will be removed in a future release. use --logLevel info
WARN  deprecated: --debug was deprecated in Hugo v0.114.0 and will be removed in a future release. use --logLevel debug
Watching for changes in /Users/me/Code/automate.fedramp.gov/{archetypes,content,layouts,package.json,static,themes}
Watching for config changes in /Users/me/Code/automate.fedramp.gov/config/_default, /Users/me/Code/automate.fedramp.gov/themes/uswds-hugo/config.yaml, /Users/me/Code/automate.fedramp.gov/go.mod
Start building sites … 
hugo v0.125.6-69ede10edcd539380914bbee58d4d32953dd8b43+extended darwin/arm64 BuildDate=2024-05-05T10:52:52Z VendorInfo=gohugoio

INFO  build:  step process substep collect files 41 files_total 41 duration 122.435041ms
INFO  build:  step process duration 122.760459ms
INFO  build:  step assemble duration 1.74575ms
INFO  deprecated: .Site.IsServer was deprecated in Hugo v0.120.0 and will be removed in a future release. Use hugo.IsServer instead.
INFO  build:  step render substep pages site en outputFormat html duration 118.732542ms
INFO  build:  step render substep pages site en outputFormat rss duration 11.981875ms
INFO  build:  step render pages 59 content 43 duration 131.919292ms
INFO  build:  step postProcess duration 25.541µs
INFO  build:  duration 256.865625ms
INFO  static: syncing static files to / duration 1.049955333s

                   |  EN   
-------------------+-------
  Pages            |   59  
  Paginator pages  |    0  
  Non-page files   |    0  
  Static files     | 2750  
  Processed images |    0  
  Aliases          |    0  
  Cleaned          |    0  

Built in 1051 ms
Environment: "development"
Serving pages from disk
Running in Fast Render Mode. For full rebuilds on change: hugo server --disableFastRender
Web Server is available at http://localhost:1313/ (bind address 127.0.0.1) 
Press Ctrl+C to stop
DEBUG Received System Events: [CHMOD         "/Users/me/Code/automate.fedramp.gov/content/documentation/ssp/4-ssp-template-to-oscal-mapping.md"]