e2e: disable coverage upload

[csuzhangxc]

What problem does this PR solve?

the coverage files merge tool can't work in Golang v1.21 now, so we choose to disable it in release-1.5

✗ make gocovmerge
GOBIN=/home/ubuntu/go/src/github.com/pingcap/tidb-operator/bin/ GOOS=linux GOARCH=arm64 go install github.com/zhouqiang-cl/gocovmerge@latest
go: downloading github.com/zhouqiang-cl/gocovmerge v0.0.0-20190125174600-5256314471af
go: finding module for package golang.org/x/tools/cover
go: downloading golang.org/x/tools v0.26.0
go: toolchain upgrade needed to resolve golang.org/x/tools/cover
go: golang.org/x/[email protected] requires go >= 1.22.0 (running go 1.21.3)
make: *** [Makefile:180: gocovmerge] Error 1

What is changed and how does it work?

Code changes

• Has Go code change
• Has CI related scripts change

Tests

• Unit test
• E2E test
• Manual test
• No code

Side effects

• Breaking backward compatibility
• Other side effects:

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation

Release Notes

Please refer to Release Notes Language Style Guide before writing the release note.

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