Add missing placeholder keys

[VandalByte]

Description

This PR adds the missing placeholder keys available in borg into vorta.

While I was working on it I noticed dt.utcnow() was deprecated, so I replaced it with the new preferred method.

Output of dt.utcnow() [DEPRECATED]

2025-04-12 18:25:42.922358

Output of dt.now(timezone.utc) [NEW]

2025-04-12 18:25:42.922358+00:00

Which according to google is a full ISO 8601 timestamp, and according to borg docs, borg accepts the ISO 8601 format.

Related Issue

Closes #1954

Motivation and Context

Vorta currently raises an "Error in archive name template" when users include valid Borg placeholders such as {utcnow}, {pid}, {borgversion} etc. in the archive name. These placeholders are documented in borg should be supported. Adding support for those placeholders should address the error.

How Has This Been Tested?

Tested it by running the application build.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have read the CONTRIBUTING guide.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

I provide my contribution under the terms of the license of this repository and I affirm the Developer Certificate of Origin.

[VandalByte]

Had a small import error, but it's been fixed.

[m3nu]

Thanks! Looks like a reasonable change.