Allow for date replacement when running release-please

[aaronware]

When releasing a version my team finds it useful to provide the date of release within our readme.md files (and extra-files as well).

It would be great to provide the date within replacements for use within files (being updated) in addition to also doing the other version bumps.

Additional information
Example: Our README.md has the following

<!-- x-release-please-lastest-version -->
### Latest Release [1.2.3] 09-02-2022
<!-- x-replease-please-end -->

It would be great to bump that date to the date of the latest merge within this action instead of doing another action for this step.

[aaronware]

This is an older task from the github action that was requested to be moved to the parent project.

See https://github.com/google-github-actions/release-please-action/issues/602

[chingor13]

I could see this being added to https://github.com/googleapis/release-please/blob/main/src/updaters/generic.ts (which is used by the extra-files config.