[release-v2.6] [DOC] Add 2.6.1 breaking change to Upgrade page #9391

Job	Run time
Build	1m 45s
Run integration tests (test-e2e)	20m 19s
Lint	3m 0s
Run integration tests (test-integration-poller)	9m 19s
Run integration tests (test-e2e-deployments)	13m 9s
Run integration tests (test-e2e-serverless)	6m 48s
Run Unit Tests (test-with-cover-pkg)	3m 5s
Benchmark	9m 25s
Run Unit Tests (test-with-cover-tempodb)	15m 57s
Build technical documentation	21s
Run Unit Tests (test-with-cover-tempodb-wal)	10m 48s
Run Unit Tests (test-with-cover-others)	12m 5s
Check jsonnet & tempo-mixin	45s
Vendor check	1m 25s
	1h 48m 11s

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