feat(conf): Add migrations.ttlSecondsAfterFinished to chart template

[ashman1984]

What this PR does / why we need it:

Adds migrations.ttlSecondsAfterFinished as an option in the chart template for allowing removal of migrations containers after a specified time.

The use of ttlSecondsAfterFinished was first discussed here, however the TTL mechanism was in beta at the time.

Special notes for your reviewer:

Checklist

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

• [] PR is based off the current tip of the main branch.
• Changes are documented under the "Unreleased" header in CHANGELOG.md
• New or modified sections of values.yaml are documented in the README.md
• Commits follow the Kong commit message guidelines

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}