Tighten docs compat: drop pre-v7 lower bounds

The docs were migrated in the previous commit to v7 / SciMLBase v3 /
RAT v4 syntax (sol.u[i] indexing, prob_func(prob, ctx) signature,
controller-object-based PID args, lazy=Val{false}(), bare tableau
names, precs on linsolve, etc.). None of that is valid on the old
majors, so allowing them through [compat] is misleading — a resolver
that picked the old major would silently produce example code that
errors.

Tightened:
- DiffEqBase:          6.106, 7   -> 7
- DiffEqCallbacks:     3, 4       -> 4
- OrdinaryDiffEq:      6.76, 7    -> 7
- OrdinaryDiffEqBDF:   1, 2       -> 2
- OrdinaryDiffEqCore:  1, 2.0, 3.1, 4 -> 4
- RecursiveArrayTools: 3, 4       -> 4
- StochasticDiffEq:    6.70, 7    -> 7

SciMLBase stays at 3.1; other deps untouched.

Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com>

Author: ChrisRackauckas

docs/Project.toml

@@ -64,8 +64,8 @@ DASSL = "2"
 DDEProblemLibrary = "0.1"
 DataFrames = "1.4"
 DelayDiffEq = "5.52"
-DiffEqBase = "6.106, 7"
-DiffEqCallbacks = "3, 4"
+DiffEqBase = "7"
+DiffEqCallbacks = "4"
 DifferentialEquations = "7.14"
 Distributions = "0.25"
 Documenter = "1"
@@ -80,18 +80,18 @@ ODEInterfaceDiffEq = "3"
 ODEProblemLibrary = "0.1, 1"
 Optimization = "3, 4, 5"
 OptimizationNLopt = "0.2, 0.3"
-OrdinaryDiffEq = "6.76, 7"
-OrdinaryDiffEqBDF = "1, 2"
-OrdinaryDiffEqCore = "1, 2.0, 3.1, 4"
+OrdinaryDiffEq = "7"
+OrdinaryDiffEqBDF = "2"
+OrdinaryDiffEqCore = "4"
 Plots = "1"
-RecursiveArrayTools = "3, 4"
+RecursiveArrayTools = "4"
 SDEProblemLibrary = "0.1, 1"
 SciMLBase = "3.1"
 SciMLOperators = "0.3, 0.4, 1"
 SimpleDiffEq = "1"
 SparseConnectivityTracer = "0.6, 1"
 StaticArrays = "1"
 SteadyStateDiffEq = "2.4"
-StochasticDiffEq = "6.70, 7"
+StochasticDiffEq = "7"
 Sundials = "4.11.3, 5"
 Unitful = "1"