HADOOP-19099. Add Protobuf Compatibility Notes

[slfan1989]

Description of PR

JIRA: HADOOP-19099. Add Protobuf Compatibility Notes.

In HADOOP-18197, we upgraded the Protobuf in hadoop-thirdparty to version 3.21.12. This version may have compatibility issues with certain versions of JDK8. We will document this situation in the index.md file of hadoop 3.4.0 and inform users that we will discontinue support for JDK8 in the future.

How was this patch tested?

For code changes:

• Does the title or this PR starts with the corresponding JIRA issue id (e.g. 'HADOOP-17799. Your PR title ...')?
• Object storage: have the integration tests been executed and the endpoint declared according to the connector-specific documentation?
• If adding new dependencies to the code, are these dependencies licensed in a way that is compatible for inclusion under ASF 2.0?
• If applicable, have you updated the LICENSE, LICENSE-binary, NOTICE-binary files?

[hadoop-yetus]

🎊 +1 overall

Vote	Subsystem	Runtime	Logfile	Comment
+0 🆗	reexec	18m 16s		Docker mode activated.
			_ Prechecks _
+1 💚	dupname	0m 0s		No case conflicting files found.
+0 🆗	codespell	0m 0s		codespell was not available.
+0 🆗	detsecrets	0m 0s		detect-secrets was not available.
+1 💚	@author	0m 0s		The patch does not contain any @author tags.
			_ branch-3.4 Compile Tests _
+1 💚	mvninstall	48m 18s		branch-3.4 passed
+1 💚	mvnsite	0m 26s		branch-3.4 passed
+1 💚	shadedclient	86m 42s		branch has no errors when building and testing our client artifacts.
			_ Patch Compile Tests _
+1 💚	mvninstall	0m 15s		the patch passed
+1 💚	blanks	0m 0s		The patch has no blanks issues.
+1 💚	mvnsite	0m 16s		the patch passed
+1 💚	shadedclient	37m 47s		patch has no errors when building and testing our client artifacts.
			_ Other Tests _
+1 💚	asflicense	0m 37s		The patch does not generate ASF License warnings.
		148m 12s

Subsystem	Report/Notes
Docker	ClientAPI=1.44 ServerAPI=1.44 base: https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-6607/1/artifact/out/Dockerfile
GITHUB PR	#6607
Optional Tests	dupname asflicense mvnsite codespell detsecrets
uname	Linux 91649e547e73 5.15.0-94-generic #104-Ubuntu SMP Tue Jan 9 15:25:40 UTC 2024 x86_64 x86_64 x86_64 GNU/Linux
Build tool	maven
Personality	dev-support/bin/hadoop.sh
git revision	branch-3.4 / dd83069
Max. process+thread count	540 (vs. ulimit of 5500)
modules	C: hadoop-project U: hadoop-project
Console output	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-6607/1/console
versions	git=2.25.1 maven=3.6.3
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[Hexiaoqiao]

LGTM. +1.

[slfan1989]

@Hexiaoqiao Thanks for the review!