Provide an AWS lambda layer for v7

[andreiborza]

Problem Statement

Currently, we publish our AWS Lambda layer under the name SentryNodeServerlessSDK and each release we bump the version number by one (e.g. for SDK release 8.2.1 the current version number for the layer is 243).

This worked fine for us in the past, but now with v8 out, we do not want to overwrite the current v8 layer with a v7 layer when we do SDK releases for v7.

Solution Brainstorm

We should create a new lambda layer named SentryNodeServerlessSDKV7 and exclusively publish to it when doing v7 releases.

SentryNodeServerlessSDK should always remain the layer name for the latest major version of the SDK, with previous majors getting their own layer names.

### Tasks
- [ ] https://github.com/getsentry/sentry-javascript/pull/12099
- [ ] https://github.com/getsentry/sentry-docs/pull/10071
- [ ] Update runtime to `node-v7` and evaluate impact on craft and sentry-release-registry; then update docs to use v7 arn dropdown

[andreiborza]

Closing this since v7 is lower prio and we can probably live with the current state of the docs.