Add generic deprecation messaging for customisation points at the end of the webpack build

[Johennes]

We're planning to eliminate customisations in favor of https://github.com/matrix-org/matrix-react-sdk-module-api. To aid the transition, let's include appropriate messaging in the output of the webpack build.

Acceptance criteria

• The message shows at or towards the end of the output (so that it doesn't just fly past)
• The message is generic in terms of timeline and mentions no concrete date for removal of customization points