website: update developer mode tutorial for v1.14.0

[ardislu]

As discussed in #29747, eth.coinbase no longer returns an address in v1.14.0+. This PR updates the developer mode tutorial to:

• Replace eth.coinbase with eth.accounts[0]. This change makes the example code work in v1.14.0+ and is also backwards compatible with previous versions of geth.
• Update all references of "coinbase account" to "developer account".

I also bumped example snippets/wording to reflect current values where applicable (e.g., block period of 12 instead of 13, updating the Remix provider from "Injected Web3" to "Custom - External Http Provider").

---

There were also some other small inaccuracies that I did not update because they require other changes in the code and/or don't greatly affect the overall tutorial:

1. I did not update example snippet of the initial logs shown by Geth when initializing developer mode because the logs in v1.14.0 currently show the following inaccuracy:

INFO [05-10|19:34:24.737] Starting Geth in ephemeral dev mode...

[...]

 3. A random, pre-allocated developer account will be available and unlocked as
    eth.coinbase, which can be used for testing. The random dev account is temporary,
    stored on a ramdisk, and will be lost if your machine is restarted.

[...]

2. The image for remix-deploy.png shows the old "Web3 Provider" environment, but Remix renamed this environment to "Custom - External Http Provider" in the current version. I'm not sure where to update this image, it's not in the repo.

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[fjl]

The images are here: https://github.com/ethereum/go-ethereum/tree/website/public/images/docs

[ardislu]

Thank you! Just pushed a commit to update all the Remix images in the developer mode tutorial so they reflect the current UI.