fix: Use starkbank/ecdsa 2.1.0

[simoheinonen]

Fixes

This PR bumps the starkbank/ecdsa version to ^2.1.0 so we get rid of deprecations.

• fixes https://github.com/sendgrid/sendgrid-php/issues/1108

Checklist

• [x] I acknowledge that all my contributions will be made under the project's license
• [x] I have made a material change to the repo (functionality, testing, spelling, grammar)
• [x] I have read the Contribution Guidelines and my PR follows them
• [x] I have titled the PR appropriately
• [x] I have updated my branch with the main branch
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] I have added the necessary documentation about the functionality in the appropriate .md file
• [x] I have added inline documentation to the code I modified

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