chore: bump versions for 7.17

[mdelapenya]

What does this PR do?

It bumps versions for 7.17-SNAPSHOT, using 7.16 as stale version for elastic-agent upgrade tests.

The hashed version used in .stack-version corresponds to the current stack version for 7.17, obtained from https://artifacts-api.elastic.co/v1/search/7.17-SNAPSHOT/elastic-agent?x-elastic-no-kpi=true

Important to say that we created 7.17 branch from 7.16 first.

Why is it important?

7.17 version is out there, and we need to support that maintenance branch.

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)

How to test this PR locally

Verified that the versions exist with this command:

TAGS="integrations" TIMEOUT_FACTOR=3 LOG_LEVEL=TRACE DEVELOPER_MODE=true ELASTIC_APM_ACTIVE=false  make -C e2e/_suites/fleet functional-test
...
INFO[2021-12-21T10:48:50+01:00] Pulling Docker images...                      images="[docker.elastic.co/beats/elastic-agent:7.17.0-SNAPSHOT docker.elastic.co/beats/elastic-agent-ubi8:7.17.0-SNAPSHOT docker.elastic.co/elasticsearch/elasticsearch:7.17.0-SNAPSHOT docker.elastic.co/observability-ci/elastic-agent:7.17.0-SNAPSHOT docker.elastic.co/observability-ci/elastic-agent-ubi8:7.17.0-SNAPSHOT docker.elastic.co/observability-ci/elasticsearch:7.17.0-SNAPSHOT docker.elastic.co/observability-ci/kibana:7.17.0-SNAPSHOT docker.elastic.co/kibana/kibana:7.17.0-SNAPSHOT]" platform=linux/amd64
...
{"status":"Status: Downloaded newer image for docker.elastic.co/beats/elastic-agent:7.17.0-SNAPSHOT"}
...

Related issues

• Closes Create 7.17 maintenance branch #1916
• Requires chore: retire 7.15 adding 7.17 #1938

[elasticmachine]

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• Start Time: 2021-12-21T09:55:15.726+0000

• Duration: 33 min 44 sec

• Commit: c0a9ba3

Test stats 🧪

Test	Results
Failed	0
Passed	210
Skipped	0
Total	210

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