[close #369] *: Upgrade to go1.21

[pingyu]

What problem does this PR solve?

Issue Number: close #369

Problem Description: build: CD failed

What is changed and how does it work?

Upgrade to go1.12

Code changes

• No code

Check List for Tests

This PR has been tested by at least one of the following methods:

• Unit test
• Integration test
• Manual test

cd cdc
docker build . -f deployments/tikv-cdc/docker/Dockerfile

cd cdc
docker build . -f deployments/docker/Dockerfile 

Side effects

• No side effects

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation
• Need to be included in the release note
• No related changes

To reviewers

Please follow these principles to check this pull requests:

• Concentration. One pull request should only do one thing. No matter how small it is, the change does exactly one thing and gets it right. Don't mix other changes into it.
• Tests. A pull request should be test covered, whether the tests are unit tests, integration tests, or end-to-end tests. Tests should be sufficient, correct and don't slow down the CI pipeline largely.
• Functionality. The pull request should implement what the author intends to do and fit well in the existing code base, resolve a real problem for TiDB/TiKV users. To get the author's intention and the pull request design, you could follow the discussions in the corresponding GitHub issue or internal.tidb.io topic.
• Style. Code in the pull request should follow common programming style. (For Go, we have style checkers in CI). However, sometimes the existing code is inconsistent with the style guide, you should maintain consistency with the existing code or file a new issue to fix the existing code style first.
• Documentation. If a pull request changes how users build, test, interact with, or release code, you must check whether it also updates the related documentation such as READMEs and any generated reference docs. Similarly, if a pull request deletes or deprecates code, you must check whether or not the corresponding documentation should also be deleted.
• Performance. If you find the pull request may affect performance, you could ask the author to provide a benchmark result.

(The above text mainly refers to TiDB Development Guide. It's also highly recommended to read about Writing code review comments)

[codecov]

Codecov Report

Merging #370 (6bd40a1) into main (0837ce5) will increase coverage by 0.4726%.
Report is 2 commits behind head on main.
The diff coverage is 74.0740%.

Additional details and impacted files

@@               Coverage Diff                @@
##               main       #370        +/-   ##
================================================
+ Coverage   61.0955%   61.5681%   +0.4725%     
================================================
  Files           240        241         +1     
  Lines         20283      20241        -42     
================================================
+ Hits          12392      12462        +70     
+ Misses         6771       6658       -113     
- Partials       1120       1121         +1     

Flag	Coverage Δ
br	60.5201% <ø> (-0.0549%)	⬇️
cdc	62.0549% <74.0740%> (+0.7176%)	⬆️

Files	Coverage Δ
cdc/cdc/kv/testing.go	100.0000% <ø> (+98.7577%)	⬆️
cdc/cdc/puller/puller.go	82.4000% <ø> (+1.1500%)	⬆️
cdc/cdc/server.go	35.1851% <ø> (-0.8338%)	⬇️
cdc/cmd/cdc/main.go	100.0000% <ø> (ø)
cdc/pkg/cmd/server/server.go	63.9751% <ø> (ø)
cdc/pkg/context/context.go	54.5454% <ø> (ø)
cdc/pkg/flags/urls.go	100.0000% <100.0000%> (ø)
cdc/pkg/httputil/httputil.go	90.4761% <100.0000%> (+5.8608%)	⬆️
cdc/pkg/regionspan/span.go	56.0975% <ø> (+5.1884%)	⬆️
cdc/pkg/security/test_util.go	0.0000% <ø> (ø)
... and 5 more

... and 19 files with indirect coverage changes