Remove unused GitHub files from backport branches

[hugovk]

We have a number of GitHub (and similar) files which are only needed in the default main branch.

I suggest we remove them from the backport branches, so it's clear they no longer need to be updated there.

These files only have effect in main:

.github/dependabot.yml
.github/ISSUE_TEMPLATE/
.github/PULL_REQUEST_TEMPLATE.md
.github/workflows/add-issue-header.yml
.github/workflows/new-bugs-announce-notifier.yml
.github/workflows/project-updater.yml
.github/workflows/stale.yml

(Note #158013 will remove add-issue-header.yml from main.)

Only the main copies of these show up in special places in the GitHub UI, but they can stay as documentation in the other branches:

.github/CONTRIBUTING.rst
.github/SECURITY.md

Also remove:

• .github/zizmor.yml - it's no longer needed: it says to ignore the dangerous trigger in documentation-links.yml but we've already deleted documentation-links.yml: Replace deprecated action with RtD app #149111.

• .well-known/funding-manifest-urls - only needed in main, to verify https://github.com/python/cpython/blob/main/.well-known/funding-manifest-urls linked to from https://www.python.org/funding.json

• .azure-pipelines/pr.yml, posix-steps.yml and posix-deps-apt.sh on 3.12/3.13 - already removed on main in GH-109408: Remove .azure-pipelines/pr.yml #122643, never backported, so can remove. Azure Pipelines only runs on pushes, not PR.

• .travis.yml on 3.10, long dead.

• .github/FUNDING.yml on 3.10, also dead, already removed elsewhere as we have an org default (Delete FUNDING.yml, since there's an organisation default #30294).