TEZ-4703: Append BUILDING.txt to root README.md

[Aggarwal-Raghav]

Removed Hadoop2.x sections and updated info about JDK21+ and protobuf version

[tez-yetus]

🎊 +1 overall

Vote	Subsystem	Runtime	Logfile	Comment
+0 🆗	reexec	8m 4s		Docker mode activated.
			_ Prechecks _
+1 💚	dupname	0m 0s		No case conflicting files found.
+0 🆗	detsecrets	0m 0s		detect-secrets was not available.
+1 💚	@author	0m 0s		The patch does not contain any @author tags.
			_ master Compile Tests _
			_ Patch Compile Tests _
+1 💚	codespell	0m 7s		The patch generated 0 new + 0 unchanged - 1 fixed = 0 total (was 1)
+1 💚	blanks	0m 0s		The patch has no blanks issues.
+1 💚	markdownlint	0m 2s		The patch generated 0 new + 0 unchanged - 20 fixed = 0 total (was 20)
			_ Other Tests _
+1 💚	asflicense	0m 26s		The patch does not generate ASF License warnings.
		9m 8s

Subsystem	Report/Notes
Docker	ClientAPI=1.54 ServerAPI=1.54 base: https://ci-hadoop.apache.org/job/tez-multibranch/job/PR-477/1/artifact/out/Dockerfile
GITHUB PR	#477
Optional Tests	dupname asflicense codespell detsecrets markdownlint
uname	Linux 34525f5bffb4 5.15.0-173-generic #183-Ubuntu SMP Fri Mar 6 13:29:34 UTC 2026 x86_64 x86_64 x86_64 GNU/Linux
Build tool	maven
Personality	/home/jenkins/jenkins-home/workspace/tez-multibranch_PR-477/src/.yetus/personality.sh
git revision	master / 22d126c
Max. process+thread count	52 (vs. ulimit of 5500)
modules	C: . U: .
Console output	https://ci-hadoop.apache.org/job/tez-multibranch/job/PR-477/1/console
versions	git=2.43.0 maven=3.9.14 codespell=2.4.1 markdownlint=0.46.0
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