Add alpine 3.21 support #4822

Job	Run time
Build Unsigned Snapshot	2m 3s
Lint	3m 52s
Performance Tests	4m 55s
Unit tests with race condition detection	2m 37s
Unit Tests	2m 14s
Load Tests	4m 37s
Integration Tests - Official Plus Images (alpine, 3.20, r32, alpine, /nginx-plus/agent)	6m 24s
Integration Tests (ubuntu, 22.04)	9m 42s
Integration Tests - Official Plus Images (alpine, 3.19, r31, alpine, /nginx-plus/agent)	6m 14s
Integration Tests (redhatenterprise, 9)	8m 32s
Integration Tests - Official Plus Images (debian, bookworm, r32, debian, /nginx-plus/agent)	6m 29s
Integration Tests - Official OSS Images (bookworm, stable, debian)	6m 16s
Integration Tests (alpine, 3.19)	6m 33s
Integration Tests - Official Plus Images (debian, bookworm, r31, debian, /nginx-plus/agent)	7m 8s
Integration Tests - Official OSS Images (bookworm, mainline, debian)	6m 16s
Integration Tests - Official OSS Images (alpine, stable, alpine)	6m 12s
Integration Tests - Official OSS Images (alpine, mainline, alpine)	6m 4s
Publish NGINX Agent v3 packages	0s
	1h 36m 8s

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