Move Social Security maximum taxable earnings parameter out of payroll folder

[MaxGhenis]

Since it also applies to self-employment earnings, it should go to irs/social_security

We may want to move all irs/payroll and irs/self_employment parameters into irs/social_security and irs/medicare.

[MaxGhenis]

PR #7245 addresses the main request by moving the Social Security wage base from gov.irs.payroll.social_security.cap to gov.ssa.social_security.contribution_and_benefit_base.

Regarding the secondary suggestion to consolidate rate parameters: this doesn't make sense because the rates are genuinely separate parameters defined in different IRC chapters:

Parameter	Rate	Statute
Employee SS	6.2%	26 USC § 3101 (FICA - IRC Chapter 21)
Employer SS	6.2%	26 USC § 3111 (FICA - IRC Chapter 21)
Self-employment SS	12.4%	26 USC § 1401 (SECA - IRC Chapter 2)

The current structure (irs/payroll/ and irs/self_employment/) correctly mirrors the IRC organization. Unlike the wage base—which is a single SSA-defined value referenced by both FICA and SECA—the rates are independently defined in separate IRC sections with different values.