Replace JSR-305 with JSpecify

[ascopes]

Closes #299.

Uses JSpecify for nullness annotations and abandons the unmaintained JSR-305 JAR.

[codecov-commenter]

Codecov Report

Merging #322 (8eb80b2) into main (f109148) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 8eb80b2 differs from pull request most recent head 3f37312. Consider uploading reports for the commit 3f37312 to get more accurate results

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@@           Coverage Diff           @@
##             main     #322   +/-   ##
=======================================
  Coverage   78.86%   78.86%           
=======================================
  Files          90       90           
  Lines        2697     2697           
  Branches      225      225           
=======================================
  Hits         2127     2127           
  Misses        514      514           
  Partials       56       56           

Impacted Files	Coverage Δ
...s/jct/assertions/AbstractContainerGroupAssert.java	100.00% <ø> (ø)
...hub/ascopes/jct/assertions/AbstractEnumAssert.java	100.00% <ø> (ø)
...s/jct/assertions/AbstractJavaFileObjectAssert.java	100.00% <ø> (ø)
...thub/ascopes/jct/assertions/ClassLoaderAssert.java	100.00% <ø> (ø)
...b/ascopes/jct/assertions/DiagnosticKindAssert.java	100.00% <ø> (ø)
...b/ascopes/jct/assertions/JavaFileObjectAssert.java	100.00% <ø> (ø)
...copes/jct/assertions/JavaFileObjectKindAssert.java	100.00% <ø> (ø)
...o/github/ascopes/jct/assertions/JctAssertions.java	100.00% <ø> (ø)
...b/ascopes/jct/assertions/JctCompilationAssert.java	50.87% <ø> (ø)
.../github/ascopes/jct/assertions/LocationAssert.java	12.00% <ø> (ø)
... and 77 more