Changes

fix(tiflow): align ci jobs with go1.25.8 (#4497)

## Summary
- bump `pipelines/pingcap/tiflow/latest/pod-merged_unit_test.yaml` to
the same Go 1.25.8-capable Jenkins image already used by `tiflow`
ghpr/presubmit unit paths
- bump `auto-update-ticdc-gomod` from `golang:1.23` to `golang:1.25.8`

## Why
`pingcap/tiflow@9fbde6ebeb0613eab6cbb372d6eeb44f58e868e6` currently
fails in two CI jobs because the runtime toolchains are older than what
the repo now requires:

- `merged_unit_test`: `compile: version "go1.25.8" does not match go
tool version "go1.25.6"`
- `auto-update-ticdc-gomod`: `go.mod requires go >= 1.25.8 (running go
1.23.12; GOTOOLCHAIN=local)`

This PR keeps the fix minimal and only updates the two affected job
definitions.

## Validation
- `git diff --check`
- parsed both modified YAML files locally via Ruby `YAML.load_file`

Signed-off-by: wuhuizuo <wuhuizuo@126.com>
 (commit: b27313a)