Update Golang version from v1.19.5 to v1.20.4

[RainbowMango]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

This PR updates the default Go version for CI and releases bots to v1.20.4.

Since we are going to update the Kubernetes dependencies to v1.27.2 which requires Go1.20+.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Karmada(v1.6) is now built with Go1.20.4.

[codecov-commenter]

Codecov Report

Merging #3565 (11c1f17) into master (2cdc72f) will decrease coverage by 0.03%.
The diff coverage is n/a.

❗ Your organization is not using the GitHub App Integration. As a result you may experience degraded service beginning May 15th. Please install the Github App Integration for your organization. Read more.

@@            Coverage Diff             @@
##           master    #3565      +/-   ##
==========================================
- Coverage   53.00%   52.97%   -0.03%     
==========================================
  Files         216      216              
  Lines       20026    20026              
==========================================
- Hits        10614    10609       -5     
- Misses       8842     8846       +4     
- Partials      570      571       +1     

Flag	Coverage Δ
unittests	52.97% <ø> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

see 2 files with indirect coverage changes

[karmada-bot]

[APPROVALNOTIFIER] This PR is APPROVED

Approval requirements bypassed by manually added approval.

This pull-request has been approved by:

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[RainbowMango]

cc @jwcesign

[jwcesign]

/lgtm