Make dco check email case-insensitive (jaegertracing#6849)

## Which problem is this PR solving?
- Resolves jaegertracing#6846

## Description of the changes
-

## How was this change tested?
-

## Checklist
- [x] I have read
https://github.com/jaegertracing/jaeger/blob/master/CONTRIBUTING_GUIDELINES.md
- [x] I have signed all commits
- [x] I have added unit tests for the new functionality
- [x] I have run lint and test steps successfully
  - for `jaeger`: `make lint test`
  - for `jaeger-ui`: `npm run lint` and `npm run test`

Signed-off-by: jinjia <[email protected]>
Signed-off-by: amol-verma-allen <[email protected]>

Author: jinjiaKarl

scripts/lint/dco_check.py

@@ -1071,10 +1071,10 @@ def process_commits(
             if not is_valid_email(email):
                 infractions[commit.hash].append(f'invalid email: {email}')
             else:
-                sign_offs_name_email.append((name, email))
+                sign_offs_name_email.append((name, email.lower()))
 
         # Check that author is in the sign-offs
-        if not (commit.author_name, commit.author_email) in sign_offs_name_email:
+        if not (commit.author_name, commit.author_email.lower()) in sign_offs_name_email:
             infractions[commit.hash].append(
                 'sign-off not found for commit author: '
                 f'{commit.author_name} {commit.author_email}; found: {sign_offs}'