Cleanup 3.0.0

[roanutil]

• Update README
• Finish availability checks for Combine for future linux support.
• Remove Swift 5.5 availability checks as minimum is now 5.5.
• Remove deprecated CustomEncoder on TopLevelEncoder

[codecov]

Codecov Report

Merging #9 (845ab74) into main (698fc3b) will not change coverage.
The diff coverage is 79.04%.

@@           Coverage Diff           @@
##             main       #9   +/-   ##
=======================================
  Coverage   74.53%   74.53%           
=======================================
  Files          27       27           
  Lines         640      640           
=======================================
  Hits          477      477           
  Misses        163      163           

Impacted Files	Coverage Δ
Sources/NetworkService/NetworkService.swift	100.00% <ø> (ø)
...es/NetworkService/NetworkServiceClient+Start.swift	72.54% <ø> (ø)
Sources/NetworkService/TopLevelCodable.swift	0.00% <ø> (ø)
Sources/NetworkServiceTestHelper/MockOutput.swift	42.85% <ø> (ø)
...rces/NetworkService/NetworkServiceClient+Put.swift	26.66% <21.05%> (ø)
...ces/NetworkService/NetworkServiceClient+Post.swift	22.22% <26.31%> (ø)
...ests/NetworkServiceTests/NetworkServiceTests.swift	43.75% <43.75%> (ø)
...tworkServiceTests/Result+NetworkServiceTests.swift	87.83% <87.14%> (ø)
.../NetworkServiceTestHelper/MockNetworkService.swift	95.83% <95.12%> (ø)
...tworkServiceTests/NetworkServiceTests+Delete.swift	96.96% <96.66%> (ø)
... and 4 more

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