Modify backup path to add root folder

[olivia-chen-github]

What problem does this PR solve?

currently ebs backup for multi az cluster will create backup path
s3://{bucket_name}/prefix-k8s-cluster. so backups for each k8s cluster don't have a root folder.

for volumebackupschedule, we saw external storage backup path folder name starting with hyphen
shared-iam/sched1/-2023-08-10t23-10-00-m-tidb-test-d-ea1-us/
shared-iam/sched1/-2023-08-10t23-10-00-m-tidb-test-e-ea1-us/
shared-iam/sched1/-2023-08-10t23-10-00-m-tidb-test-f-ea1-us/

What is changed and how does it work?

Code changes

• Has Go code change
• Has CI related scripts change

Adding a new level of folder in backup path.
e.g.
volumeBackup vb1 creates folder
s3://{bucket_name}/{backup_namespace}/vb1-k8s-cluster1,
s3://{bucket_name}/{backup_namespace}/vb1-k8s-cluster2 and
s3://{bucket_name}/{backup_namespace}/vb1-k8s-cluster3.

backup path after this change will be like
s3://{bucket_name}/{backup_namespace}/vb1/k8s-cluster1,
s3://{bucket_name}/{backup_namespace}/vb1/k8s-cluster2,
s3://{bucket_name}/{backup_namespace}/vb1/k8s-cluster3

for volumebackupschedule, this code change will remove the hyphen

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.

Reviewers: @WangLe1321 @BornChanger

[codecov-commenter]

Codecov Report

Merging #5245 (695f135) into master (ddc83e6) will decrease coverage by 0.01%.
Report is 1 commits behind head on master.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5245      +/-   ##
==========================================
- Coverage   57.76%   57.76%   -0.01%     
==========================================
  Files         246      246              
  Lines       29479    29479              
==========================================
- Hits        17029    17028       -1     
- Misses      10795    10796       +1     
  Partials     1655     1655              

Flag	Coverage Δ
unittest	57.76% <100.00%> (-0.01%)	⬇️

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