chore(deps): update golang docker tag to v1.21 (#20)

Signed-off-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>
Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

Dockerfile

@@ -1,5 +1,5 @@
 # Build the manager binary
-FROM golang:1.20 AS builder
+FROM golang:1.21 AS builder
 ARG TARGETOS
 ARG TARGETARCH