refactor(app): De-duplicate and fix version handling code

[teor2345]

Motivation

This PR cleans up a bunch of version handling code in the application, and RPCs.

Solution

• De-duplicate app_version and user_agent code, rename to build_version
  • Fix RPC tests with new argument
• Use "modified" rather than "dirty" for uncommitted changes in build version
• Split the vergen version into its own function

Related fixes:

• Make RPC testnet flag forward-compatible with additional testnets

Review

This is a low priority cleanup.

Reviewer Checklist

• Are the PR labels correct?
• Does the code do what the ticket and PR says?
  • Does it change concurrent code, unsafe code, or consensus rules?
• How do you know it works? Does it have tests?

[codecov]

Codecov Report

Merging #6996 (307be10) into main (0da2dcb) will decrease coverage by 0.05%.
The diff coverage is 72.72%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6996      +/-   ##
==========================================
- Coverage   77.49%   77.44%   -0.05%     
==========================================
  Files         310      310              
  Lines       41536    41539       +3     
==========================================
- Hits        32189    32171      -18     
- Misses       9347     9368      +21