Updated roadmap for v5.x

[bart-degreed]

This PR contains our current plans for JsonApiDotNetCore v5.x. We're closing active development on v4.x. This means we're not going to deliver the next issues for v4.x anymore:

• RFC: Optimistic concurrency #1004 because we suspect we can provide a better experience by making breaking changes.
• More helpful exception if resource has not been registered to contextgraph #414 because it is closely related to planned work for nullable reference types.
• Static Validations of the ContextGraph #170 because it was a low-impact, non-breaking change, but we feel it can wait, in favor of more important work.

[codecov]

Codecov Report

Merging #1034 (e765f30) into master (6429275) will decrease coverage by 0.36%.
The diff coverage is 98.91%.

❗ Current head e765f30 differs from pull request most recent head fa02dc6. Consider uploading reports for the commit fa02dc6 to get more accurate results

@@            Coverage Diff             @@
##           master    #1034      +/-   ##
==========================================
- Coverage   91.47%   91.10%   -0.37%     
==========================================
  Files         275      254      -21     
  Lines        7671     6894     -777     
==========================================
- Hits         7017     6281     -736     
+ Misses        654      613      -41     

Impacted Files	Coverage Δ
src/JsonApiDotNetCore/CollectionConverter.cs	97.43% <ø> (+28.20%)	⬆️
...JsonApiDotNetCore/Configuration/IJsonApiOptions.cs	100.00% <ø> (ø)
...c/JsonApiDotNetCore/Serialization/JsonApiReader.cs	100.00% <ø> (ø)
.../JsonApiDotNetCore/Middleware/FixedQueryFeature.cs	60.86% <66.66%> (+1.77%)	⬆️
...cOperations/Processors/SetRelationshipProcessor.cs	100.00% <100.00%> (ø)
...NetCore/Configuration/InverseNavigationResolver.cs	100.00% <100.00%> (ø)
...NetCore/Configuration/JsonApiApplicationBuilder.cs	100.00% <100.00%> (ø)
.../JsonApiDotNetCore/Configuration/JsonApiOptions.cs	100.00% <100.00%> (ø)
...otNetCore/Configuration/JsonApiValidationFilter.cs	100.00% <100.00%> (ø)
src/JsonApiDotNetCore/Configuration/PageNumber.cs	46.66% <100.00%> (ø)
... and 52 more

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