Merge the development deltas into a single perldelta

[book]

This covers almost all cases. The goal is that the release manager will only have the editing work left, and not have to spend time copy/pasting sections from the individual deltas into the final one.

Solves #23365.

• This set of changes does not require a perldelta entry.

[book]

There is some work needed still:

• the final version should be saved in pod/perldelta.pod
• the Platform Support section is not handled yet

[book]

I've dropped all previous changes, and went for a much simpler approach: just copy every relevant section from each development delta into the final one. This leaves more editing work to the release manager, but still removes the tedious and error-prone copy/pasting step (which was the original goal before I got carried away).

It also avoids issues when the section being copied does not look exactly as expected and might be incompletely copied into the master document.

[book]

And this new version removes unmodified template sections from the final document.

[book]

The change from the initial version which handled each section depending on the expected content (=head2, =item, etc.) to the current (just copy the whole content of each section into the master document) is motivated by the need to avoid losing a change because it does not conform to the expected format.

Another useful aspect is the removal of whole sections when they have not been modified from the template.

When preparing a blead final release, the steps will be:

1. run the script
2. edit the resulting document

This is removing toil of copying and pasting stuff from the development cycle perldeltas, allowing the releaser to focus on the actual work of copy-editing.

[thibaultduponchelle]

Thank you for that, it's resolving a release management toil

[book]

Merged as 798066b