refactor(server): rename DemoApplication to Server

[Ahoo-Wang]

• Rename DemoApplication.kt to Server.kt
• Update mainClass in build.gradle.kts from DemoApplicationKt to ServerKt
• Modify main function to use Server::class.java instead of DemoApplication::class.java

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.00%. Comparing base (7211ae0) to head (1551c5f).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #407   +/-   ##
=======================================
  Coverage   92.00%   92.00%           
=======================================
  Files           6        6           
  Lines          25       25           
  Branches        1        1           
=======================================
  Hits           23       23           
  Misses          1        1           
  Partials        1        1           

Flag	Coverage Δ
unittests	92.00% <ø> (ø)

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