Bump maven-site-plugin 3.21.0 #223

Job	Run time
Verify / ubuntu-latest jdk-17-zulu 3.9.9	1m 18s
Verify / setup-matrix	1s
Verify / ubuntu-latest jdk-17-zulu 3.6.3	32s
Verify / ubuntu-latest jdk-17-zulu 3.9.9	2s
Verify / ubuntu-latest jdk-21-zulu 3.6.3	37s
Verify / ubuntu-latest jdk-21-zulu 3.9.9	31s
Verify / ubuntu-latest jdk-8-zulu 3.6.3	32s
Verify / ubuntu-latest jdk-8-zulu 3.9.9	41s
Verify / windows-latest jdk-17-zulu 3.6.3	1m 16s
Verify / windows-latest jdk-17-zulu 3.9.9	1m 7s
Verify / windows-latest jdk-21-zulu 3.6.3	1m 9s
Verify / windows-latest jdk-21-zulu 3.9.9	1m 19s
Verify / windows-latest jdk-8-zulu 3.6.3	1m 8s
Verify / windows-latest jdk-8-zulu 3.9.9	1m 15s
Verify / macos-latest jdk-17-zulu 3.6.3	48s
Verify / macos-latest jdk-17-zulu 3.9.9	44s
Verify / macos-latest jdk-21-zulu 3.6.3	46s
Verify / macos-latest jdk-21-zulu 3.9.9	50s
Verify / macos-latest jdk-8-zulu 3.6.3	58s
Verify / macos-latest jdk-8-zulu 3.9.9	42s
Deploy / Deploy	30s
	16m 46s

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