feat: upgrade go.mod to 1.21 (#1830)

This matches what was changed in the Dockerfile

### CSpell results

CSpell: Files checked: 243, Issues found: 0 in 0 files

### GOSec results

Summary:
  Gosec  : 2.18.1
  Files  : 121
  Lines  : 13091
  Nosec  : 8
  Issues : 0

### Test Results

?   	github.com/mikefarah/yq/v4	[no test files]
ok  	github.com/mikefarah/yq/v4/cmd	0.693s
ok  	github.com/mikefarah/yq/v4/pkg/yqlib	1.023

Signed-off-by: jmeridth <jmeridth@gmail.com>

go.mod

@@ -30,4 +30,4 @@ require (
 	golang.org/x/xerrors v0.0.0-20220907171357-04be3eba64a2 // indirect
 )
 
-go 1.20
+go 1.21

go.sum

@@ -1,6 +1,7 @@
 github.com/a8m/envsubst v1.4.2 h1:4yWIHXOLEJHQEFd4UjrWDrYeYlV7ncFWJOCBRLOZHQg=
 github.com/a8m/envsubst v1.4.2/go.mod h1:MVUTQNGQ3tsjOOtKCNd+fl8RzhsXcDvvAEzkhGtlsbY=
 github.com/alecthomas/assert/v2 v2.3.0 h1:mAsH2wmvjsuvyBvAmCtm7zFsBlb8mIHx5ySLVdDZXL0=
+github.com/alecthomas/assert/v2 v2.3.0/go.mod h1:pXcQ2Asjp247dahGEmsZ6ru0UVwnkhktn7S0bBDLxvQ=
 github.com/alecthomas/participle/v2 v2.1.0 h1:z7dElHRrOEEq45F2TG5cbQihMtNTv8vwldytDj7Wrz4=
 github.com/alecthomas/participle/v2 v2.1.0/go.mod h1:Y1+hAs8DHPmc3YUFzqllV+eSQ9ljPTk0ZkPMtEdAx2c=
 github.com/alecthomas/repr v0.3.0 h1:NeYzUPfjjlqHY4KtzgKJiWd6sVq2eNUPTi34PiFGjY8=
@@ -24,6 +25,7 @@ github.com/goccy/go-yaml v1.11.2 h1:joq77SxuyIs9zzxEjgyLBugMQ9NEgTWxXfz2wVqwAaQ=
 github.com/goccy/go-yaml v1.11.2/go.mod h1:wKnAMd44+9JAAnGQpWVEgBzGt3YuTaQ4uXoHvE4m7WU=
 github.com/google/go-cmp v0.5.9 h1:O2Tfq5qg4qc4AmwVlvv0oLiVAGB7enBSJ2x2DqQFi38=
 github.com/hexops/gotextdiff v1.0.3 h1:gitA9+qJrrTCsiCl7+kh75nPqQt1cx4ZkudSTLoUqJM=
+github.com/hexops/gotextdiff v1.0.3/go.mod h1:pSWU5MAI3yDq+fZBTazCSJysOMbxWL1BSow5/V2vxeg=
 github.com/inconshreveable/mousetrap v1.1.0 h1:wN+x4NVGpMsO7ErUn/mUI3vEoE6Jt13X2s0bqwp9tc8=
 github.com/inconshreveable/mousetrap v1.1.0/go.mod h1:vpF70FUmC8bwa3OWnCshd2FqLfsEA9PFc4w1p2J65bw=
 github.com/jinzhu/copier v0.4.0 h1:w3ciUoD19shMCRargcpm0cm91ytaBhDvuRpz1ODO/U8=