Upgrade Unity to 2021 LTS

[nowsprinting]

• Upgrade Unity to 2021.3.45f1
• Remove Unity under 2021 from running tests on CI
• Remove net.tnrd.nsubstitute
• Turn off "Assembly Version Validation"
• Install NSubstitute v5.3.0
• Install NSubstitute.Analyzers.CSharp v1.0.17
• Install NUnit Analyzers v3.3.0

[github-actions]

Code Metrics Report

	master (c4fc931)	#17 (695c11c)	+/-
Coverage	36.1%	36.4%	+0.2%
Code to Test Ratio	1:3.3	1:3.3	+0.0
Test Execution Time	5m4s	4m44s	-20s

Details

  |                     | master (c4fc931) | #17 (695c11c) |  +/-  |
  |---------------------|------------------|---------------|-------|
+ | Coverage            |            36.1% |         36.4% | +0.2% |
  |   Files             |               29 |            29 |     0 |
  |   Lines             |              492 |           489 |    -3 |
  |   Covered           |              178 |           178 |     0 |
+ | Code to Test Ratio  |            1:3.3 |         1:3.3 |  +0.0 |
  |   Code              |             1325 |          1321 |    -4 |
  |   Test              |             4470 |          4470 |     0 |
+ | Test Execution Time |             5m4s |         4m44s |  -20s |

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Reported by octocov