Forms: switch namespace from `jetpack/contact-form` to `jetpack/form`

Internal reference p1HpG7-l0G-p2

> I’d like to have a nicer namespace and switch from jetpack/contact-form to jetpack/form.

Discussion Jan 2025 (internal ref p1737635493924209-slack-C086RGTJT1D ):

> When we load a post, we get the `html` (`post_content`) and parse it through the core API ( in `@wordpress/blocks`). This is where some other operations take place, like block invalidation etc..
>
> So if a block is called `some name` and we wan't to change it to `another name` (let's say `new name`) we would need to make sure the parsing of the content doesn't invalidate the blocks already in posts with the `some name` name and instead handle it gracefully and update it with our new name. That handling takes place before the final parsing, so by changing the name we don't end up with invalid blocks.
>
> This handling is actually needed in at least another place, which is the `template` [prop of blocks](https://developer.wordpress.org/block-editor/reference-guides/block-api/block-templates/).
>
> So, unless there is another way that I'm not aware of (I don't think so though..), in order to rename 3rd party blocks we would need to have a filter somewhere related to [convertLegacyBlockNameAndAttributes](https://github.com/WordPress/gutenberg/blob/a411f333ae717961749714064b35c373f2f0b6c1/packages/blocks/src/api/parser/convert-legacy-block.js#L11C17-L11C52) to add more blocks to convert.

Start with the @wordpress/blocks parser entry point and read convertLegacyBlockNameAndAttributes, then review the block template prop handling described in the issue. Determine how existing jetpack/contact-form content can be recognized and migrated to jetpack/form without invalidation; done means old posts and templates parse correctly under the new namespace.

Written by the indexing model from the issue text.