adjust CO2 tax phase-in for diffCurvPhaseIn2Lin

[orichters]

Purpose of this PR

• see https://github.com/remindmodel/development_issues/issues/295

Type of change

• Minor change (default scenarios show only small differences)

Checklist:

• My code follows the coding etiquette
• I performed a self-review of my own code
• I explained my changes within the PR, particularly in hard-to-understand areas
• I checked that the in-code documentation is up-to-date
• I adjusted forbiddenColumnNames in readCheckScenarioConfig.R in case the PR leads to deprecated switches
• All automated model tests pass (FAIL 0 in the output of make test)
• The changelog CHANGELOG.md has been updated correctly

[orichters]

Superseded by #1723