[openstreetmap/openstreetmap-website] Add and unignore `.ruby-version` file to ensure consistent dev Ruby (PR #6333)

[Nicholas La Roux]

### Description

Adds and unignores `.ruby-version` file to ensure a consistent development Ruby for all contributors. Doing so also helps push contributors onto newer rubies which can sometimes be quite helpful, such as with the latest [Ruby 3.4.5](https://www.ruby-lang.org/en/news/2025/07/15/ruby-3-4-5-released/) which addressed a ton of GCC 15 gem compilation error issues.

Additionally, having a `.ruby-version` file has become a somewhat standard practice in Ruby development such that the [chruby CLI tool](https://github.com/postmodern/chruby) and the [ruby-lsp VSCode extension](https://marketplace.visualstudio.com/items?itemName=Shopify.ruby-lsp) expect it to exist. There's even a proposal to spec the `.ruby-version` file format over in rubygems: https://github.com/orgs/rubygems/discussions/7074.

**Why is `.ruby-version` currently ignored?**

Looks like it may have been arbitrarily ignored as a side change in https://github.com/openstreetmap/openstreetmap-website/commit/14ac1babc2517320a2c90fa9b4ac36a5a6e68018 over 10 years ago.

### How has this been tested?

Yes it has been tested. See the screencasts below.

**Before** (`master` branch)

https://github.com/user-attachments/assets/55d7605f-c4ce-4914-92ea-8c9ac0319375

**After** (this branch)

https://github.com/user-attachments/assets/cefcc1e8-4156-4d1a-bde8-f427646346f4

Can also see that the later shell (above screencast i.e. this branch) properly uses Ruby 3.4.5 immediately by default thanks to the `.ruby-version`, which is highly preferable.
You can view, comment on, or merge this pull request online at:

  https://github.com/openstreetmap/openstreetmap-website/pull/6333

-- Commit Summary --

  * Add and unignore .ruby-version file to ensure consistent dev Ruby

-- File Changes --

    M .gitignore (1)
    A .ruby-version (1)

-- Patch Links --

https://github.com/openstreetmap/openstreetmap-website/pull/6333.patch
https://github.com/openstreetmap/openstreetmap-website/pull/6333.diff

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