RFR: 8303410: Remove ContentSigner

[Eirik Bjorsnos]

On Wed, 1 Mar 2023 11:46:26 GMT, Lance Andersen <lancea at openjdk.org> wrote:

> For your spec changes, I might borrow the format I used in [JDK-8193757](https://bugs.openjdk.org/browse/JDK-8193757)

I updated the Specification section to use the list-style format from JDK-8193757. I also included a mention of the jarsigner deprectated options section.

I'm assuming the Specification section should focus on externally visible changes, not implementation-internal removals.

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PR: https://git.openjdk.org/jdk/pull/12791