[OOB] Upgrades 'dotnet-core' to '4.2.21'

[SvcGitHubPATagentoperatorimages]

Automated OOB update requested by SvcGitHubPATagentoperatorimages.

Agent: dotnet-core
Version: 4.2.20 -> 4.2.21

[github-actions]

Images built:

dotnet-core: ghcr.io/contrast-security-oss/agent-operator-images/agent-dotnet-core@sha256:b16e50485fb56c0ed79d0ea2566e2011907a636a2e87145fe8add0217a2f956f
dotnet-framework: ghcr.io/contrast-security-oss/agent-operator-images/agent-dotnet-framework@sha256:e884930f748ecb10f0e9ddb80f0b8c0c680fd1a12cf99af1fb169ca39ea5fb11
java: ghcr.io/contrast-security-oss/agent-operator-images/agent-java@sha256:acc3c83ebfd8f2e8ed88f83637eff1297992cb2d52e14f315ffa84f724e9f1e3
nodejs: ghcr.io/contrast-security-oss/agent-operator-images/agent-nodejs@sha256:ddb256de0cc90926bbffd45d0730dd556f45aaac192ccc83eb2b0e488295706b
php: ghcr.io/contrast-security-oss/agent-operator-images/agent-php@sha256:2f0787f25f512a3761aa34bcc06b9240bc36faa2a817ba9b5014a81984345099
python: ghcr.io/contrast-security-oss/agent-operator-images/agent-python@sha256:474c1df036ed38986eff7720e5d96118ed6cfa84d519f9bd0c7816d24840e645