Correct historical VAT rates in German regime

[rilla]

Summary

While working on #750 I stumbled upon some inaccuracies in historical tax rate definitions for the German regime (the COVID end date seemed to be off by a year and that raised my attention).

I searched the OECD.org database but couldn't find the dataset originally used when this regime was introduced in #221, so I researched authoritative sources and found that the original implementation had wrong dates, wrong rates, and a spurious entry in the reduced rate history.

Changes

Standard rate (Regelsteuersatz)

Entry	Was	Now	Source
Post-COVID restoration	2022-01-01	2021-01-01	Destatis: "The reduced rates applied from 1 July to 31 December 2020."
Pre-1998 rate	16%	15%	Bundestag WD-4-040-10 p.4: "Erhöhung des allgemeinen Umsatzsteuersatzes von 15% auf 16% ab dem 1.4.1998" ("Increase of the general VAT rate from 15% to 16% as of 1.4.1998")
1998 increase (new entry)	missing	1998-04-01 → 16%	Bundestag WD-4-040-10 p.4: "Erhöhung des allgemeinen Umsatzsteuersatzes von 15% auf 16% ab dem 1.4.1998" ("Increase of the general VAT rate from 15% to 16% as of 1.4.1998")

Reduced rate (ermäßigter Steuersatz)

Entry	Was	Now	Source
Post-COVID restoration	2022-01-01	2021-01-01	Destatis: "The reduced rates applied from 1 July to 31 December 2020."
Historical rate	2007-07-01 → 7% and 1993-01-01 → 5%	1983-07-01 → 7%	BMF: "Dieser ermäßigte Steuersatz liegt seit 1983 konstant bei 7%" ("This reduced tax rate has remained constant at 7% since 1983")

Sources

Replaced the generic OECD tax database link with three German government sources that directly back the rates defined in the file:

• Federal Ministry of Finance (BMF) — "100 Jahre Umsatzsteuer in Deutschland" (2019)
• German Bundestag Research Service — WD 4-040-10 (2010)
• Federal Statistical Office (Destatis) — "Effect of the VAT Reduction"

Secondary sources

For a consolidated overview of all historical German VAT rates in one place, see:

• zinsen-berechnen.de: Historische Entwicklung der Mehrwertsteuer in Deutschland.

Didn't add this to tax_categories.go since it's not an official government source

Pre-Review Checklist

• Opened this PR as a draft
• Read the CONTRIBUTING.md guide.
• Performed a self-review of my code.
• Added thorough tests with at least 90% code coverage.
• Modified or created example GOBL documents to show my changes in use, if appropriate.
• Added links to the source of the changes in tax regimes or addons, either structured or in the comments.
• Run go generate . to ensure that the Schemas and Regime data are up to date.
• Reviewed and fixed all linter warnings.
• Been obsessive with pointer nil checks to avoid panics.
• Updated the CHANGELOG.md with an overview of my changes.
• Marked this PR as ready for review.

And if you are part of the org:

• Requested a review from Copilot and fixed or dismissed (with a reason) all the feedback raised.
• Requested a review from @samlown.