fix: use the right branch for 7.17 backports

[mdelapenya]

What does this PR do?

It uses 7.17 as the branch used by mergify when a backport:7.17 label is added to a PR

Why is it important?

7.17 was not receiving backports and it was because of that

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have run the Unit tests (make unit-test), and they are passing locally
• I have run the End-2-End tests for the suite I'm working on, and they are passing locally
• I have noticed new Go dependencies (run make notice in the proper directory)

[elasticmachine]

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Build stats

• Start Time: 2022-01-19T16:18:08.803+0000

• Duration: 5 min 33 sec

• Commit: 46b512c

Test stats 🧪

Test	Results
Failed	0
Passed	19
Skipped	0
Total	19

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Tests succeeded.

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