ci(agent): update agent image tag in readme on release

[aroberts87]

What this PR does / why we need it:

when a new version of the agent is released, make sure we increment the default value for image.tag mentioned in the readme file in addition to the values file

Checklist

[Place an '[x]' (no spaces) in all applicable fields. Please remove unrelated fields.]

• Title of the PR starts with type and scope, (e.g. feat(agent,node-analyzer,sysdig-deploy):)
• Chart Version bumped for the respective charts
• Variables are documented in the README.md (or README.tpl in some charts)
• Check GithubAction checks (like lint) to avoid merge-check stoppers
• All test files are added in the tests folder of their respective chart and have a "_test" suffix

Check Contribution guidelines in README.md for more insight.