sql: use txn.NewBatch instead of &kv.Batch{}

[rafiss]

This will make these requests properly passes along the admission control headers.

informs #79212
Epic: None
Release note: None

[cockroach-teamcity]

This change is 

[yuzefovich]

Maybe add a linter for this too (like simple text based, over pkg/sql* ignoring tests)?

[rafiss]

Maybe add a linter for this too

There is a usage in pkg/sql/tenant_gc.go that I don't know how to address. I was planning to reach out to the multi-tenant team to see if that can be addressed separately.

There are also some in pkg/server that I don't know what to do about.

[rafiss]

I also found usages like this one:

cockroach/pkg/sql/gcjob/table_garbage_collection.go

Lines 245 to 251 in 5acb999

	var b kv.Batch
	b.AdmissionHeader = kvpb.AdmissionHeader{
	Priority: int32(admissionpb.BulkNormalPri),
	CreateTime: timeutil.Now().UnixNano(),
	Source: kvpb.AdmissionHeader_FROM_SQL,
	NoMemoryReservedAtSource: true,
	}

It seems like it would be better to have a linter or runtime assertion to check that AdmissionHeader is always set, one way or another.

[fqazi]

Reviewed 11 of 11 files at r1, all commit messages.
Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @irfansharif and @yuzefovich)

[rafiss]

tftr!

bors r=fqazi

[craig]

This PR was included in a batch that was canceled, it will be automatically retried

[craig]

Build succeeded:

• Bazel Essential CI (Cockroach)

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